Dental x-rays look fine but molars hurt when i chew. one in particular (a crown) is pretty sensitive but both sides hurt...why?

See a specialist: Could be many things from bruxing to occlusal dysfunction. Start with an endodontist to evaluate the nerve status of the teeth.

Sadly difficult : To diagnose without the benefit of examining you and looking at the x-ray. Most common complaint for sensitivity on crown tooth is premature contact. Make sure that the occlusion is correct in movements. It may be that the tooth has a crack (cracked tooth syndrome). Many conditions are not visible radiographically so your dentist will need to do some diagnostic tests to determine the cause.