What went wrong and why did she cheat? (question for former cheaters)

When I first met my girlfriend (2 years now) I was head over heels. She was amazing by all the standards. However I came to find out that nothing's perfect and only after a year of our relationship is when I finally started seeing the clear picture of who my girlfriend really is and was for the past year that I was dating her. And what I came to find out was upsetting. The biggest disappointment was to find out that she cheated me within a month of us dating and she kept it from me for a year. I had my suspicions and confronted her a few times about it but she lied, lied and lied until I had some real proof. I trusted her so much that even having all the signs pointing to her to actually having intercourse, I still thought that maybe things never went passed making out. I was wrong. She slept with this guy, who was her friend back from High School and was residing in the city she went to visit. And she said the reason she didn't tell me was because it was none of my business. she also said she didn't feel guilty about it at all because at that point she wasn't sure whether she loved me or not. This is where I'm confused. What has happened to her?

Before she went on that trip, she was head over heels for me as well (or so it seemed). I even read her personal blog and I saw that she had an entry in there about her trip. She said something along the lines of being so in love with me that leaving me for 4 days seemed like eternity. And that all her friends tell her to go out, have fun and catch some tail while on the trip, all she really wants to do is be with me. The night she slept (it was her first time hooking up with him) with her old high school friend we talked 3 times. Third time is when I arose suspicion and to be honest my worst expectations after that call that night came true regardless of me trying not to think about it. My girlfriend told me during the 3rd call that she was pretty drunk and that she was taking shots of tequila (she's told me before that tequila is what makes her clothes come off) because her friend was doing really well at their craps table and they were being served free. that's when I told her that I knew where this was going...she told me that not this time and that I have nothing to worry about. I went to bed, when I woke up I saw that she texted me at 4 am and told me she loved me and missed me. Next time I talked to her was 10 a.m. She sounded like hell. She told me her mom called her 17 times because she was afraid they were going to miss the flight. And she didn't go to bed until 7 am.

So wtf happened in 3 hours? I mean I know some sex happened but did my gf's friend take advantage of the fact that she was drunk? Or how do these things happen? She told me it wasn't planned...which her text at 4 am proves. So I'm thinking he made his move and she went along with it Because drunk?