Me and my ex broke up after almost a year and I found out a few days ago that she had sex with some guy from school like two weeks after we broke up. We ended up getting back together but now all I think about when we do something is I wonder if she did this with him and I really want to beat him up for taking advantage of her like that right after we broke up but I don't know what to do?

I wouldn't be angry at him, you guys broke up and not only that she 'chose' to have sex with him. You should maybe think about what went wrong for your relationship to break down and take it like a man.

Why did you break up? Who dumped who? Whose idea was it to get back together? Did you get back together before or after she told you about having sex with some dude? How old are you both?

I seriously doubt she was taken advantage of so leave the guy alone for sure. Wasn't his fault they had sex.

Most likely it wasn't guy it was her. If she went on to have sex with another guy after only two weeks after the break up you didn't mean much to her. Think of it. How can someone have sex with another person right after a brake up. They never really loved you.