So I am a medical student, and I was assigned to do my 5 week surgery block at an away hospital with a classmate. We spent a lot of time with each other because we lived together in the same house and worked on the same team. Over the first 2 weeks we talked a lot and had some late night couch conversations and cooked and baked together and had a good time. So after 2 weeks we hooked up, then for the next 3 weeks we spent every night together too. Eventually the surgery rotation ends and we go back home, and I ask her how we want to move forward. There is a complicating factor that I knew about prior, which is that she had been in an unserious situationship with an objectively attractive guy for a few months. Given that, I wanted to give her time to figure out if she wanted to be with me or not. So we decided we would carry on, non-exclusive. 3 weeks go by, and every possible night except for 2 we spend time with each other and sleep over, and I feel like things are going well and cute and I like her and she likes me. Last night I go to a friends birthday dinner then get drinks after. When I get home, I see her location is not at her place. I wake up in the morning, and it's still at that place. At this point I get a little pit feeling in my stomach. She stays at the place until 2p. We have a date night planned, and that's very fun and enjoyable. When we get back home, I initiate the convo by telling her I want to be vulnerable, I've realized over the past few weeks how much I have liked you, and I only want you to want me and be with me tldr I want to be exclusive. She says that she had been feeling the same way, and 2 weeks ago had told the other guy that she was going to be moving on because she liked me. I was feeling good at this point and asked when the last time she saw him was. She asked if I saw her location last night, I said yes. She confirmed that she had gone to his apartment and hooked up last night. So now I'm stuck and wondering what to do. AITAH if I cut things off with her now because I'm not liking how this situation makes me feel, even though we were in a period of non-exclusivity?