update***!! boyfriend cheating!!!

Update:
So tonight once he got home from work I decided to tell him what I had found and to try and talk about it as adults. I didn't want to argue or fight because it's stressful for the baby and I. I brought it up very calmly and it immediately backfired. Well he pretty much said he didn't do anything wrong and at that point we weren't even technically "together". Well okay then so he got his things and he left. This is extremely hard on me, he threatened to get full custody of our son when he's born and is treating me like it's all my fault. He's not sorry for anything. I have a long road ahead of me and didn't want this to happen but I know even as a single mom I have to do what's best for myself and the only thing I should be concerned about is making sure I take care of my baby boy. 
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Part 1
Im so confused and upset right now because I'm not sure how I should feel about this!!! Boyfriend left his phone at home today when he left for work and I figured okay we've been together since December 2015 never had any problems so I figured if I went through it I wouldn't find anything. Well this girl and him have known each other for a couple years and we're supposedly best friends and I never thought much of it. He said they were intimate one time way before our relationship happened and that it was too awkward and never went through. I never saw him ever texting or talking to her though and I'm most upset because in the snapchats he was sending her he was also sending me the same thing at the same time!!!! He even told me to come cuddle and watch cops with him (at the time we didn't live together yet) even worse we had planned out a whole trip up to the mountains at the beginning of February literally 2 and a half weeks after this was happening. I just can't believe I never knew and that I just found out. The thing I'm worried about is what else there might be that I don't know. She was obviously sending him pictures it just makes me sick. Idk how to bring this up though because I'm so upset but it's been over a year now am I too upset about this??? I'm 7 months pregnant right now with his child and worried that there's been other things in the past he's deleted and I just don't know about. Somehow I guess he forgot to delete this one. I'm so worried now I feel like I can't trust him because something tells me if he's done it once he's probably done it or going to do it again. Help I'm too emotional for all this😭😭