Girls... I need your help on this one!?

My girlfriend and i have been together for a year and she is amazing in every way, she has never lied to me, never cheated, never talks to other guys, and just is incredible. The problem is that before we dated she used to drink and im not a fan of it so i told her that i didn't want her to drink and she completely changed for me. With that said the other night she told me she felt bad about something and needed to tell me. So of course i was worried. She told me when she stayed the night at her friends house she finally gave in and drank a little, not to get drunk but tipsy since her friends really pushed her to and she just finally gave in after a year, after that she said her best friend was laying in bed with her after it all and asked her to kiss and she randomly did (she is not a at all) and then the touched each others you know whats for like 30 seconds then realized that she was completely losing it and stopped and said what the hell are we doing. They agreed it was sick and that it was just a weird spontaneous thing and never wanted to speak of it again. However i felt terrible and hurt cause it took her a week to finally tell me. We talked about it and she said im perfect and she never meant to hurt me she said if she could go back she would just stop before the drinking ever started and come to me and she saw how hurt i was and that i was crying and asked me to forgive her cause she loves me more than anything. I told her I was really mad but I understood people make mistakes and she just started bawling cause she thought she was losing me :( We are okay now and have talked about it over and over but I still feel bad about it. Should I be reacting this harshly or should I just feel relieved that this is the only time she has lied to me and hurt me and accept her apology and move on?