“No, I’m sure I don’t need to,” Sweetie Belle said with an excited grin. The pang of guilt from lying to you was quickly drowned out by the prickling satisfaction of being naughty and getting away with it that filled the back of her head. The soft confines of the car seat held her in its comfortable embrace as she wiggled against the restraints, the excitement flaring up in her as she realized she didn’t even need to bother holding herself upright. She was strapped in tight and snugly, just like a real foal.
And just like a real foal, it was alright for her to not tell when she had an achingly full bladder. Sweetie Belle pushed her hips forward, just to feel the tingling sensation in her filly parts again as the crotch strap insistently made her aware of the thick diaper resting between her hind legs, the infantile crinkle underlining her every movement. She had never felt like this before, but the way her heart raced as the diaper practically begged her to soil herself like a foal made her feel like it was alright. Her hips instinctively worked to grind her against the seat again, and it felt so good, there was no way she could have done anything against it.
“Alright then,” you comment absentmindedly, trying to get the damn mirror lined up. “Well, we’re gonna stop somewhere in a few hours, so if anything comes up in between … well, you know, like whatever …” It’s not like you hadn’t seen to protection for your seats anyway.
The engine sprang to life as you turned the key.

---

“Sweetie Belle, are you alright?” you interrupted your story with a glance in the rear view mirror. You didn’t have a problem with letting her have some of her weird little fun at pretending to be a foal, that she had so spectacularly failed to disguise as embarrassment, but the way she looked and intensity she squirmed around with was starting to worry you.
“Uh huh…” came the reply from Sweetie Belle. She had barely said anything else in response to your constant babbling, except maybe for the occasional negative “nuh uh…”.
She was panting heavily. Tiny droplets of sweat stood on her otherwise pristine white coat. Her face was burning, not only because she was hot, but also because her skin was prickling all over from the excess of pleasure that her young body wasn’t capable of handling. The electrifying jolt of lust had first exploded from under her belly the moment the car engine had first roared to life half an hour ago, imposing its visceral vibrations upon her without her having a say about it. It was evident that she didn’t even care though, seeing how she had squirmed in her seat, closed and spread her hind legs, arched her back and stretched her tummy out, moved her hips back and forth, relaxed into and pushed against her restraints, all of that with barely any control over her body as her instincts worked hard to be able to intensify the vigorous massage given to her by the car seat and the diaper.
“Uh, if you say so,” your words snapped her out of her funk.
“No, really I’m fine,” Sweetie Belle said breathlessly. She desperately wanted to go back to doing what she was doing, but she hated the thought of you worrying about her. “it’s just a little … um,” she said before a brilliant idea of how to placate you came to her, “I think I need to pee.”
She knew damn well how far the next stop still was. It was all she had been thinking about when she had held herself back, saving up for the ride. She just needed something to fire her up again.
“Uh, I guess,” you said, “you should just, you know … just go.”
That did the trick.

---

Her little heart beat faster in Sweetie Belle’s chest as she observed how all of your attention went back to the (straight, empty) road. Snapped back to reality, she was now attentive enough to feel the urge of her bloated bladder beseeching her for attention. One of her fore hooves gently caressed her soft tummy, her fluffy coat tickling her as she explored below her belly button, the stinging internal tension intensifying as she only slightly pushed down on all the pee pee inside her.
It was easy for Sweetie Belle to relax her muscles with the padded restraints enveloping her, keeping her in a comfortable sitting position even as she let herself sag, resting her head back into her bouncy, pink and purple mane. She tried her best to savour the feeling of her inner muscles giving in to the pressure as the familiar prelude to relieving herself elicited screaming bouts of what she didn’t know were desire and lust exploding forward from her fillyhood. Recollections of her pitiful past attempts to fulfill her secret wishes shot through her head. The little droplets that she had been able to release into the towels and pieces of cloth had been teasing her as she had rubbed them against herself before having to painfully hold herself back lest anypony should notice. None of that would happen now that she had the encouragement of her diaper. She was way past the point of even caring.
The first drop of pee tickled sensuously as it rolled down Sweetie Belle’s little slit. The hot liquid soaked into her crotch fur as she let it dribble into her diaper, staining both the soft cloudy fluff and her creamy white filly parts with blotches of yellow. Somewhere inside her, a desperate voice insisting that what she was doing was wrong was drowned out in the intense thrill of getting away with it, as she let her flow become a full-blown stream of pee. Her little body worked to empty itself without any hindrance on Sweetie Belle’s part as her diaper began doing its job of distributing her urine across its resources.
Sweetie Belle lost herself in pure bliss as she hung in her car seat relieving herself, splashes of hot liquid drenching her crotch with her pee moistening her hindquarters as her diaper took care of her needs the best it could. With a detached fascination she assessed that she would fill her diaper to its limits with how much she was imposing upon it. There was no shame or worry, only the prickling sense of primal relief filling the back of her head.
Her lips were glistening in spittle as they permitted a small moan of pleasure. Sweetie Belle was basking in the sensation of urinating on herself in an infantile way, her diaper soaking it all up and reassuring her that it was okay for her to not go to the bathroom like a foal. Even after the last drops had made it out of her empty bladder, she still relished the feeling of her lower body resting in her diaper thoroughly soaked from front to back.
She would have felt satisfied relief, had not the good feeling she enjoyed so much returned, coursing through her body with a ferocity she had never felt before. She tried to lift her forehooves off her belly, the bloated, sagging image of her diaper drawing her attention against her will, but a jolt of prickling pleasure shooting through her had her paralyzed before long. Even her tiniest movements had the primal lust flare up inside her as she tried to work her hooves towards her hot diaper.
There was no holding back when they finally arrived. Sweetie Belle had just wanted to touch her diaper from the outside, but when the pressure of her hooves made itself apparent against her sensitive parts, her thighs had instinctively pressed together.
With eyes shut tightly and her face contorted in climax, she inhaled sharply as she was pushed over the edge, a powerful orgasm shaking her little filly body to the core. Her nether lips convulsed in their hot and wet confines, clenching together around protrusions of the soiled diaper that were pushed between them, as if they were trying to press it against her feel-good spot any harder than her hooves already were. She issued forth soft “Nnnnnnnh…”s and “Aaahhhh…”s as wave after wave of pleasure coursed through her, fulfilling a primordial desire deep in her soul she hadn’t known existed.
You worriedly glanced into the mirror at the commotion behind you to find Sweetie Belle tensed up, her forehooves pressed tightly between her closed hind legs, panting and sweating heavily. “Sweetie Belle,” you started saying with panic building up in your voice, “if you’re really feeling that bad then --”
Your eyes shot wide open as realization hit you like a hammer to the face, the car swerving on the other lane before getting back on track. The way she just hung there, her face glistening in sweat, contorted in that particular way. Your panic transformed into a different kind of creeping fear. You knew what that expression indicated. You had seen it often enough on Rarity.

Oh shit! Rarity!

How could you ever explain this to her? That mare had always had a deep connection to tidiness and cleanliness. What would she say if she knew that her own little sister had had what was possibly her first orgasm right there in your car seat while wearing a used diaper? And that it was YOUR fault for putting her it on her? You were fucked, that’s what.

You considered your options.