What's the real issue?

I've been with my SO for a year and a half now. We're engaged and expecting a son March 3, 2016. In August, I found out he was texting and exchanging photos with 3 different females. He begged for a second chance and promised to prove himself worthy. Things were ok for a short while, but then he started causing drama. Small things set him off and caused arguments. He began going through my phone and questioning me all the time. He was always "joking" about me having another man. This morning we had sex and then I got up and showered for work. When I came in the room to get dressed he was staring at me with a weird look. I repeatedly asked what the problem was but got no response. As soon as I pulled into the parking lot at work, I got a message saying "Smh. You be on some other shit". I am totally lost. Again I asked what this is all about but got the run around. I let him know that I'm not a child and refuse to play guessing games. When he wants to argue he always does it at the beginning of my work day. It's so annoying. I'm trying to figure out what the real issue is. I'm not sure if he's scared about the baby, cheating, tired of the relationship, or just finally showing who he truly is.