What would you do in this situation?

My boyfriend recently found out his ex girlfriend is pregnant, now keep in mind I am fine with him talking to every one of his exes EXCEPT her. We had a rough past with her and agreed to leave it and her behind us. It took me a year and a half to get over and he knows how much it affected me. None of us have mentioned anything about it to the other in a year so I thought he never would talk about her again. We had a fight yesterday and he came back home last night and we started talking. He brings up that his friend mentioned that my boyfriends ex is pregnant. Anyways his phone is missing and while he's asleep I find it and see that she messaged him back (all the other messages were deleted) saying bye and that if he (my boyfriend) ever needs anything then he can talk to her. I was pissed because first, he lied about it, second we agreed to put that and her behind us, and third he acted like he didn't know what I was talking about when I threw the phone at the wall (which hit him) and woke him up. I tell him to leave and he takes (pulled him from my arms) our 3 week old son with him. He comes back in a couple hours because our son is refusing formula. Then I proceed to hit (not hard at all) my boyfriends hand because there was a fly on it and apologize. My boyfriend then full on slaps me in the face. I started screaming at him to get out. I just want to make sure I wasn't in the wrong here. 

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