What to do about it?

So my girlfriend told me some guy she hung out with kissed her when she was leaving, after a goodbye hug. So, I asked her who and she didn't tell me. She said "What are you going to do with that info?" I'm like "Nothing, I just want to know" (All right, would you want to know if you were in the same situation as me?) If you were going to do something or not? Every one would want to know, anyone. So I was like "If you don't tell me we are done" and she is like "I'm not telling you" So it happened, I broke up with her. After doing so, I wanted her back. So I had to freaking beg her to take me back and she wouldn't, very very sad. So it happened 2 days ago, I have been crying and crying so much. Anyways, see look. She could have cheated on me. Or told it how is was. So I am going to have to ask her friend, because after that she talked to her friend(girl)for like 20mins, so maybe she told her what happened. God, right now, I have no idea what the hell to do. I am scared, pissed and sad. That's the story.