I had a root canal for one tooth then put a crown on it..why does it hurt now?

A few thoughts..: The bite may be high, there may be a recurrent infection, there may even be a fracture. So hard to tell over the internet, sight unseen. Call your dentist for an appointment to be evaluated.

Call dentist: It's possible that the crown is slightly high and is hitting prematurely when you bite your teeth together. Call your dentist to have the bite checked.