Why is my wonderful boyfriend being a jerk all of a sudden?

Ok. Here's the deal. He has a ring and I know it. Every since the day I heard he got, he has been acting like a jerk. Not romantic, like leading up to a proposal. Its almost like he wants to pick a fight, concenrate on out differences and make the most of them. I think he is wanting me to break up or something. Its the first time I've ever even considered it. He told me I'd gained weight one day. One day he said he was having second thoughts b/c we had different tastes in music and movies. After he spouts this, he goes right back into acting like we're practically engaged, which we are. I think it has to do with nerves. But why would he be a jerk?