You may have cracked a rib (it is possible), but more likely is that you pulled a muscle or popped a rib slightly out of joint; they're not connected by more than cartilage and ligaments in the back to allow for expansion when you breathe. It's the same thing that's happened to me a couple times in the last year or so (especially if the pain is in the back, near your spine/kidneys).

A good chiropractor can pop the rib back into place, or it will eventually work its way back in. In the meantime, it's pretty agonizing, I know. :/