Worried

My fiancé, me, our kid, and his friend just got a place together. Well since we have all moved in, his buddy has had his girlfriend over all week and I feel like my fiancé is being weridly nice to her. He's never done this with any of his girlfriends expect this one. I mean he's made her food, plays with her kid ( only takes care of our daughter once a day), asked me today if he should go ask her if she wants to come with us to go shopping for food, and then I was on the phone with him on his lunch break and was telling him why she has been staying here all week (his buddy told me so I told him) and he says that she's always welcomed here. I just find it very uncomfortable how friendly he is with her. I don't know if I'm worried for nothing but i feel like I'm not there or anything. It makes me upset. I don't know if I should talk to him or what.