She had sex with someone else....?

My girlfriend and I of 5 years broke up about 7 months ago, we still talk and had feelings about each other. We just got back together and I found out from a friend while we were on a break she had sex with another guy. I guess he came over got her drunk and they had sex. Shes the only person Ive had sex with and the fact that when i asked her about it she said no dont you trust me? then later she told me the truth after i kept pressing her about it. What should I do im so lost, I thought sex was just gonna be like between me and her, It was something personal we both shared. I could understand if we broke up for good but the fact that we were still talking and trying to mend hurt feelings and she goes and screws another guy. What am i suppose to do? Maybe im just bitching....