Is my girlfriend gonna cheat on me?

Well we're both in 8th and going to 9th. Who ex is also in 8th but doesn't go to our school. We've been dating for three days and doing the whole boyfriend girlfriend thing. Huggin, holding hands, etc. And she and her boyfriend broke up like an hour before we started goin out. I just found out this yesterday so kinda mad about that. So we were talkin today and I asked her wats up. She was honest and said talking to her ex. So I said do you still like him, not caring about the answer cause I'm not the jealous type. She said yea, and I also found out she broke up with him cause he was a jerk and pressured her to have . But she still likes him. And he was calling me a wimp, and out of the blue, my girlfriend starts talkin about his muscles. Wtf right? But I didn't really care she brought it up, just didn't wanna talk about his muscles. I don't really like talking about other guys muscles. Does any guy? And so she had to go and I just think that she might cheat on me for her and don't wanna be the guy getting screwed over. So wats happening? Should we break up? Or wat? Any help would be great. Thanks