I think you'll have to see a dentist or your physician to figure this out. It could be a tooth - that might be the best explanation, actually - or it could be jaw problems (ie TMJ). You might want to check out Health Central's TMJ section and some of the home remedies that can relax those jaw muscles. Good luck!
TMJ diagnosis - http://www.healthcentral.com/chronic-pain/tmj-38789-5.html
TMJ Treatment - http://www.healthcentral.com/chronic-pain/tmj/treatment.html