HELP relationship advice

Okay so my boyfriend and I have a 9 month old son together. For about a month we had a falling out and weren't really communicating.. no one said things were over but we just weren't how we normally were. About a month later he reached or saying he wanted to work on things, I agreed for the fact we have a son. Things have been great until I decidied to look at his phone last night, during the time we weren't together he had sex with another woman and I am TORN apart especially for the fact it's someone whose just been waiting to get with him for as long as I have known him. He doesn't know I know, but I saw a video he took on his phone and all her nudes she sent him. I don't know what to do, should I be upset? I don't know because we weren't speaking during the time but I think if he really loved me would he have been able to have sex with someone else. Should I bring it up? Help please we were together 4 years beforehand and I am completely heart broken. I also saw on his Snapchat he was snapping a girl that he knows I don't like, he said she snaps him randomly he doesn't reply. I also don't really know how to bring it up as I can't even speak it without crying 😭