I (31m) met my gf's (25f) friends this week and I'm 100% sure I matched with her best friend on a dating app very recently who is in a longterm relationship.

Met my gfs best and closest friend today and the second I saw her my stomach dropped. I am 100% positive I matched with her on a dating app very recently. I deleted my apps so I can't go back and look but I am certain it was her. She literally couldn't look at me and everyone there could notice something was wrong and she even made an excuse to go back into a restaurant we were at for basically the entire brunch. I've only been seeing my gf for a month or so now and it's a dream come true but I have no idea what to do. I dont have any hard proof, I JUST started dating this girl and im absolutely bonkers for her but I've never been so sure about something in my entire life. Do i confront her? or should i tell my gf? I already asked her if any of her friends are in an open relationship and it's possible they dont tell anyone but my gf and this girl have been best friends for well over a decade so I feel like she would have told her. Could really use some advice!

should also mention that our conversation on the app was VERY sexually charged and i got the sense she was drunk and wanted to meet up that night but I declined.

tldr: matched with my gf's best friend very recently on a dating app who is in a long term relationship and i'm not sure how to proceed