I've been having severe pain "episodes" (they lasts 10-20minutes) on the left side of my face. it often leaves my left side of my face numb and stiff. i've been to the dentist - no root canal problems. it's very debilitating. what can be the cause?

Facial Pain: It certainly does not sound like a root canal problem. There can be a number off different causes, including bruxism, TMJ disorder, or even a neurological issue. You should probably begin with an ENT physician for a complete medical history and exam.

"Episodes": Prudent to be evaluated to rule out Tic Douloureux or Trigeminal Neuralgia. Evaluation by a Neurologist would be a good idea.