My boyfriend was talking to a guy he had a thing with in the past

I really need some advice for this situation.Tonight my boyfriend and I decided to stay in and just watch TV instead of going out with friends. We were in bed cuddling and he was on his phone. He was messaging somebody on Snapchat and I happened to glance at his screen that was right in front of me and I saw the words "my cock inside of you" in the message he received.Immediately I moved my arm from under him and asked him what that was. He said nothing. I asked him again and then stated what I had seen. He still said nothing so I then said his name and asked him a third time what that was. He told me that it wasn't anything and that it was a guy that he had a thing with in the past. He said he had sent a chat regarding a post of this guy's dog and said he thought his dog was cute. Me mentioned he also said that he wanted to see his dog in person and then he said that gut said something along the lines of he'd have to charge for that. I'm assuming my boyfriend asked what that charge was and that lead up to the message I saw.He explained to me that it wasn't what it looked like and that there wasn't anything going on between them and that he doesn't even have this guy's number, he just has him on social media. He also said he didn't anticipate that response from him.I don't know what to think. I tried to let it go but it's bothering me so much. Why was he even messaging this guy in the first place if he's with me, why did he ask what the charge was and give this dude attention? Based off of what he told me, It seems to me it was clear what this guy was getting at. I didn't see the messages myself but he kept his phone close to him all night. I wasn't going to go through it, but I thought that was really odd because usually he'd keep it on the charger but instead he kept it with him when he went to sleep.My boyfriend just moved out of town and lives about 100 miles away from me now. I drove there to see his new place and to help him figure out which bus lines he needs to take to get around. I even called out of work today to help him do that.I feel so hurt, betrayed, and like I can't trust him anymore. I don't know what to do. I tried to stay the night and hoped that it would blow over but it was just awkward all night. He apologized to me and explained he didn't know that would happen.I ended up leaving after he fell asleep because I couldn't sleep either and I felt so uncomfortable around him. I've never felt that way around him. I don't even feel like talking to him, but I feel like I'll need to, but idk what else to say. I don't know what to do.I'm sorry this might seem scattered, but any advice is greatly appreciated.