Why is he being a jerk?

Yesterday, I imed my best guy friend, asked him how his day was as usual, and he said it was bad. When I asked if he wanted to talk about it, he said no, I don't feel like having a conversation. I said then why are you having a conversation with your girlfriend? He said that was different and when I asked why, he was a completed and said cuz it is, now bye. Pretty stupid since I've known him way longer than his gf, who is only an online gf who he has nvr met before. We said we could tell each other anything and now he's not talking to me. I'm rly pissed off at him, but I want us to be good again. I think he might be mad at me cuz I have a new bf and he used to or still likes me, but idk. Need help :(