What is your opinion in this trust situation?

I did not want to put my relationship business out there, but I need some opinions. Well, it’s kind of an issue that’s been bothering me for a while now and I need some more views or opinions on it. One day, my girlfriend was mad at me and then, after all of it was over, explained to me that she spoke to her friend (a guy) about them having sex. She tells me she would never do something like this to me, that she is loyal to me. She tells me it just came up and they spoke about it. So someone answer me this, what stops me from thinking that one day, I will make her mad to the point I’ll drive her to do something crazy, like I don’t know, maybe having sex with this guy? What stops me from thinking that one day, in all the fury or whatever that she will go through with doing something that will end up in hurting me. I trust her, I do, I don’t want to have a reason not to, but what can I do? When I have a reason to doubt, I will think the first thing that pops in my head.