Need some serious advice...

Okay ladies... so I'm currently 30 weeks pregnant. I'm 20 and my babies father is 24. We were living together and we got pregnant. Due to some problems during my pregnancy I moved back home with my dad. I need your honest opinions. Him and I had broke up when I moved out and we would still talk everyday still text and tell each other we loved each other and what not. Well, one day he invited me to come stay the weekend with him at the house I use to live at. So I did, we were on very good terms. He was in the shower, and me feeling insecure I looked through his phone and i went on his Snapchat and he had been messaging one of his ex's he dated a long while ago. And the conversation was pretty friendly. And it got to the point where they were talking about sexual stuff... not sexual stuff with each other, just sexual things in general. And then they would randomly talk about her and her new boyfriend and she would even ask about how I was doing during my pregnancy... I was so hurt I immediately left his house. This was about 2 months ago and I forgave him for it, because he said he didn't mean it like it looked. I just need to know some opinions on this. Do you think he was in the wrong messaging his ex while we were NOT together but we were trying to work out our issues and what not... I can't seem to get it out of my mind. I just really need some help. What would you do in this situation??