I(27M) walked in on my girlfriend (25F) cheating. I have zero proof. Actions?

We've been together just under a year. Things kinda got intense real quick. I love her very much, but not in an overly obsessive way. We've been having some minor issues the last few days, and I was hoping we'd be able to work it out because we're going on vacation in a couple days. I'd rather die than cheat on someone and the idea of cheating on me would make me never be able to trust someone again. It's one of the biggest crimes you can commit in a relationship as far as I'm concerned. I give 100% trust to everyone until they abuse it, which she did.<br><br>So, I walk into my apartment after work, and hear noises. They immediately and abruptly stop. I am absolutely certain it was her having sex with someone, but I have zero proof. There is someone in my bathroom (she does this sometimes when she has to immediately use the bathroom). When she comes out, it's one of her friends from work. This friend recently went through a bad breakup and was going to come over and talk to my girlfriend tonight.<br><br>The friend is visibly upset, has puffy eyes, and is breathing heavily. She hastily grabs her things and leaves immediately. My girlfriend tries to lie to me and say she was just emotional and crying. I play along a little and ask her a couple questions. Her story has some inconsistencies, but there's nothing I can nail her on. I say okay, well maybe we can go out to eat tonight and cheer you up, as this friend apparently left in a sad state. My girlfriend says she's not up to it and is just going to go to bed.<br><br>It's been a long day and I'm tired myself, so I don't even press it and go straight to bed. The next morning she leaves without saying goodbye and I realize there's still wet spot on the sheets from the night before. We have never had sex in my bed. It's something sandbagging me because I know damn well she couldn't have gotten that there herself.<br><br>What I'm wondering is, what should I do now? That happened late yesterday afternoon and this is the first time I've really been able to think about the situation. To me, it's over. Even if I couldn't prove it, which I can't, I just can't get rid of the feeling that she did cheat. In fact, I'm sure of it. So my only option here is to leave?<br><br>I was really hoping to go on this vacation with her because I've been saving up money for two years to afford it and my time off is already approved. It's also where I was planning on proposing. If it were to happen like that though, I'd essentially be proposing to someone who I'm 100% sure cheated on me. I know it's a dumb question to ask because cheating is cheating, but am I just overreacting at this point?