I had a root canal 3 weeks ago. 4 days ago a started experiencing lower left jaw pains and sharp pains. the x-rays show no infections or abnormality ?

See endodontist: You didn't mention if the jaw pain was same side as root canal. I'm assuming it is. Not all root canals work. If your root canal was done by a general dentist, see opinion of root canal specialist (endodontist). If the root canal was done by an endodontist, and he/she says no problem, get an opinion from a second endodontist.

See your dentist: If you have severe pain or pressure in your jaw that lasts more than a few days, contact your dentist or endodontist for re-evaluation. Not all problem teeth display symptoms you can detect on x-rays.