Any ideas on why i'm getting jaw pain (close to ear) while playing harmonica?

Tmj disorders : Apparently you are suffering from TMJ disorders . You might be a bruxer or you are grinding your teeth while asleep or you are a clincher . Get your TMJ and surrounding muscles examined by a well trained dentist in that field .The pain you are getting is due to muscle contractions while you are playing

TMJ: TMJ problem. See TMJ expert for evaluation. Any dentist can be a TMJ expert with the proper training and experience. Most commonly, oral surgeons, prosthodontists, and orofacial pain specialists. Ask your MD, your dentist and your dental society for referrals.