Had root canal done on #19 a month ago. wore a temp crown for 3 weeks, felt great with no pain. then had my porcelain crown put on now with pain?

Porcelain crown: It's possible that the bite needs to be adjusted on the porcelain crown. Call your dentist for an appointment to have the crown and the bite checked.

Bite or cement: Have the dentist that made the crown check the bite and look for bits of cement that might be causing the discomfort

Most likely: Most likely, all that is necessary is a slight adjustment in the bite, not only when you are biting down, but also in excursions. There are other less likely scenarios, such as a fractured root, but see your dentist for an evaluation and bite adjustment first.

Simple adjustment: the most likely cause is the new crown being too high in certain spots when you bite down. A simple bite adjustment should take care of the problem.