So for a bit of context:

I met this cute girl a few months back and we seemed to really hit it off, I slept with her after the first date even though we'd met through mutual friends before. She said she knew I was an asshole but still wanted to get to know the 'real' me. Since then we've been dating exclusively (sort-of) and it is going pretty good, great sex and she would always offer to pay for the meal or whatever. Not really noticed any "red-flags" up until now.

I've never really cared who she hung around with, I knew she was pretty close friends with a few guys from her history degree since the start of college but didn't really think anything of it. We're talking the other day and she mentions that she's actually slept with the one of the guys from her course twice (8 months apart) but he wasn't looking for anything serious and he always sleeps around with girls.

I was quite clear that I wasn't comfortable with this and she assured me he's just a friend. The thing is I've noticed she's been at his house doing uni work with him and his housemate multiple times a week. She'll even be messaging me through Snapchat or whatever while shes quite clearly at his place.

Usually if a girl was disrespecting me like this I'd just hard next her. The reason why I'm a bit confused is because everything else is great, we'll have sex multiple times at her/my place, she'll cook me breakfast and wake me up with blow-jobs etc. She seems to worship me like I'm a God when I'm with her in person, which is why its really fucking with my head a bit.. Any suggestions on how I approach this? Do I just break it up now before it gets worse?