My (26M) girlfriend (25F) went on an "innocent" vacation without telling me

My girlfriend went to Miami for 3 days to visit her old friend she hadn't seen in years. But she never said anything to me about it until she messaged me from the airport saying "Hey, I'm currently in Miami right now, and I'll see you Sunday!" I was like "what?!! How did you not tell me!! Why didn't you mention anything about planning a trip??" Then she just responded with "I didn't think it was necessary to tell you. She and I haven't seen each other in years, so I had to go." I just let it go and didn't want to make a big deal out of it.<br><br>Fast forward to this past weekend, Sunday, when she got back, she showed me pictures of her trip. Nothing out of the ordinary. Until she clicked on one and I noticed a guy standing behind her with a smirk on his face. I asked "Who's the dude?" She immediately looked nervous and said "Oh that's my friend's boyfriend." I was like "he sure as hell ain't her boyfriend because he was eyeing you down and didn't even look at the camera." She insisted that he was her friend's boyfriend and even showed me pictures of her and her friend's "boyfriend" to prove to me she was telling the truth. I let it go. But I just couldn't let it go in my head that she was hiding something.<br><br>On Monday, I asked her straight up "Can I get the names of the people you met there?" And she said "Why?" I said "Because I wanna see for myself if that dude was her boyfriend or not." She was like "I don't see how that's necessary." I said "Well, it is if you got nothing to hide and want to keep trust in this relationship." She was like "Fine" and gave me her friend's Instagram and her "boyfriend's" Instagram. I looked through his Instagram, and lo and behold he has pictures of him with his ACTUAL girlfriend. I sent the pic to her and was like "you're not gonna sit there and lie to me after I just found a picture of him with his actual girlfriend." She was like "I'm not his girlfriend, okay!! I'm not dating him!! We're just friends!!" I asked "Did you hook up with him?? Was that the whole reason you went on this trip??" She was like "No!! I did not!!" But lo and behold I found a pic of her wearing a bikini with the same background as his story post from the day before with the location of Miami. I said "You're not gonna lie to me!! This was his story post from the day before you left!! That means you were in his apartment!! That he took this picture!!" She was like "He was just taking the picture!!" I said "Yeah sure. You were wearing a bikini in his apartment. That makes total sense." She was like "I'm telling you we're just friends!! I don't know how many times I have to tell you!!" At this point, I just left the house we were in and went back to my moms and have not talked to her since. <br><br>I just feel like she got caught and was like "Oh, he'll believe me if I keep insisting we're just friends." I just don't know what to believe anymore. She went behind my back to go on a trip I knew nothing about. She was lying to me about the dude being her friend's boyfriend. And she was taking pictures with him in his apartment. It's just too much and I feel used and betrayed. I don't know what to do. I don't know if I should just break up with her or what. I just feel hurt and stupid for having to find out myself that she was hiding something.<br><br>TLDR: My girlfriend went to Miami for 3 days without telling me. I found a picture of her with a guy she claimed was her friend's boyfriend. But lo and behold he's not her friend's boyfriend and she was probably hooking up with him the whole time.