My girlfriend gave some dude head.

Okay so I was dating this girl for a few weeks, and things at school got really hectic with pledging a fraternity, being VP of my class, and doing a bunch of other stuff and I couldn’t handle a relationship at that time. I told her that we needed to break up but that I still liked her and that if things calmed down soon we’d take it from there. She had been hanging out with one of my friends a lot and they got close. I knew that kid was kind of s scum bag so I talked to him the day she and I broke up that I still liked her and vented the situation to him so that he wouldn’t try anything. A few weeks later things calm down and she and I start hanging out again. Things were going really well and she asked if I’d be interested in getting back together and I said definitely. She then told me that she had gotten fingered and given head to the guy/my friend that she was hanging out with. I felt sick to my stomach, but after talking to some friends, and hearing her appology (she used to be that type of girl but has changed a lot) I decided to take her back. I have always been scared of her past but this made it seem like it had come back which scares me. The image of her blowing this guy haunts me every day. What should I do or thing of it?

Thanks