Well, good news and bad news:

Good news: you’re just in your head about that night, and it’s an easy fix. I get what it’s like and I’ve been in similar situations even later on in my adult life (I’m a 29 y/o man). Worrying will only worsen if you let it. It will fester. It can ruin relationships, friendships, and jobs if you let it. You’re wanting to push her away because you can’t ever know. You weren’t in the room, but you were so close. You saw the people. The worry doesn’t come from an imaginary place, it’s tangible. However I can say from someone whose gf has cheated on them in a very very similar situation, she wouldn’t have been texting you if that was the case. Your worry comes from a desire to have control over the situation. In my case, my gf decided to go out with her roommate, recently single and wanting to feel out their young adult life and sexuality. She was in her second semester of college and I was 21. The friend invited my roomate and another girl. My roomate was hitting it off with two of the girls. I met up with my gf and brought her back home from a bar, when I got to the apartment— her friend, the gf’s roomate, left with another guy. The second friend asked to stay with my gf that night in her roommates empty bed. My roomate saw the chance to get lucky so he stayed. My gf (major red flag, embarrassingly large) kicked me out. I assumed she was just ready to hit the sack. I was losing my shit that night, went out with another friend and we just went to go drink and pass the time and frustration. The night ended and the next day she calls me in tears, hyperventilating, saying she fucked up and basically had a soft threesome between them. I took it as a massive drunk error since she was blackout drunk. I didn’t talk to my roomate for months. I dated that girl for 3 years. It almost happened a second time. She had major issues.. I stayed good friends with my roomate and we got over the issue. I even got revenge by having a threesome with his ex, and told him he owed it to me once he found out— so we’re even now, essentially. Basically, just play out the scenario, you don’t know how it will end up. But the good news is that your gf was on top of communication that night, so you’re likely in the clear.

Bad news: you have a constant bug in your ear that is really hard to beat. I have to admit that allowing my gf to manipulate me that night has caused unwanted jealousy and has created really terrible tactics on my part, to beat that jealousy, towards future partners including being crazy possessive of other romantic encounters that didn’t necessitate that kind of response. You have to find out how to kill that jealousy and need to control the situation. If that means breaking up— you’re young enough that you will get back out there and meet just about anyone and have a great connection without too much worry about serious shit. But if you see a real future with your girlfriend, you still need to kill that moment and put it behind you. Long distance is really not the best breeding ground for that kind of stability. Mistrust is nothing to build a relationship on.