I'm loyal so why is he this way?

My boyfriend has my car so it can get looked at I wanted to use his so I could pick up some clothes from my sister. He usually lets me use his but today is different. He said that if I leave he will call the cops and say that I stole his car. All I wanted to do wis drive 15 minutes to my sisters house to borrow some clothes? Why is it that I'm not allowed to? What did I do wrong? I'm loyal. I would never and I have never cheated on him. So why is he being this way?