In all honesty, it could be too many different things to say for certain. It could be that she's trying to come across as more 'cute' in front of you, possibly that she swears more with others because it helps her 'fit in' with the people she's around, or that she doesn't feel as comfortable around you for whatever weird reason, so is less relaxed. Knowing how long you've been together would probably help answer, but in all honesty you should just ask her. Don't assume the worst though