Me and the girl are on a break... again

I want some advice about me dealng with a problem i have with my girlfriend. We dated for 2 years and decided to break up with me cuz she needed time. But before allof this happened she was spending alot of time with this group of people she was intro diced to by this guy. SO i was kool with no problems im glad she was making friends. But during our break up for 3 months she decided to sleep with this kid after i called it out before we broke up. So after the 3 months passed she wanted to get backtogether. She waited a month to come back after she slept with him. But long story short we got back together and now since the skool year started that kid wants to hang out again in the group of friends. She say theres no feelings but i feel like something kood develope cuz she had little feelings for him during our breakup. but she wants to hang with them again and im not ok with it right now. She slept with the dood and i dont think its appropriate for her to hang with him after all of that. Its tearing me up inside. I dont ever want to tell her who she can and cant hang out with and i dont want to be controlling i just think its wrong she has no problem hanging with the person she slept with and still try to make or realtionship work. I feel likw shes choosing them over me even though she says she loves me and i love her to. But ths whole situation makes me feel aweful. SHe has never cheated on me but what she did sure feels like it.