Somethings wrong with my rabbit?

I bought two rabbits two days ago, and they are really close and get on well. One of them, Jack, has started acting weirdly. He runs around the hutch and moves really suddenly, and goes and sits in his litter box, then jumps out again. He is really jumpy and twitchy. I thought he was just scared of me, but he lets me pet him when i try to calm him down. I'm really worried he'll hurt himself or the other rabbit, so what's wrong, and what can i do?