My husband moved his girlfriend in

My husband(30 m) and me (29 f) talked about divorce about 4 months ago. Our marriage had been over for a while we just didn't want to rush the divorce process due to me having a baby a year ago and me having ppd and ppa I didn't want to make a big decision while going through that. Fast forward to now, we had both been seeing people without our divorce papers even being filed yet. I guess we thought maybe we would work it out eventually. Well he fell in love and moved her into our apartment. I feel very uncomfortable.. I tried to talk to him but I don't think he understands how it feels. I'm not mad at him for finding someone who makes him happy. I'm mad because he didn't even ask me if it was okay. He just did it. Now I'm stuck here with both of them hearing them screw all night. I'm a stay at home mom and I have plans to go back to work just so I can get the hell out of here but I need to know if I'm over reacting about this? I just feel disrespected and now our son is going to be comfused... he's only 1.

EDIT: Thank you all so much for the responses and overwhelming support. It makes me feel less crazy. I will be taking some action. Thanks everyone for all the suggestions. We rent so yes I will be able to tell the landlord and get her out I believe. But first I'll try less legal action and try talking to him again. I don't want to make a scene if I don't have to.