Husband's facebook

Two nights ago I was up quite late as I can't sleep comfortably due to my growing belly and at 4 am my husband's phone pinged so I picked it up to see who is messaging at this hour. Turned out it was a facebook message by some girl I'd never heard of. I opened the conversation and it consisted of a stream of messages by my husband, constantly asking her about one thing or the other and short, snappy replies by her. She even said things like why are you messaging me and why don't you to talk to your wife instead, to which he replied don't worry about my wife and I like talking to you. My whole world came crashing down after I read it. We were in a relationship for two and a half years and got married this year in Feb after going through so much together. We had to overcome a lot of family objections to get married but we still did it because we loved each other so I can't imagine why did he do this. I woke him up to confront him about it and he said this is what you get for going through my phone. I asked him why did he do this and he said because I like talking to her, I was so shaken I said why didn't you marry her instead then and he replied because I couldn't and can we sleep now. The next day he acted like nothing had happened and then later when I said something about it, he apologized and promised he will never do it again. But I am so shaken, I didn't expect this in a million years. I am almost 7 months pregnant, I don't know what to do or who to tell and whether to move on or not.