Headache, upper full jaw pain, can only open mouth half way then get more pressure/pain in head. seen dentist 3 weeks ago no issues. what is cause?

Sinus problems?: If you are not having any pain that you can point to an exact tooth, and/or there is no temperature sensitivity to any of your teeth, i would first have an oral surgeon take a panoramic x-ray film to make sure there are no abnormalities. If there are none, i would take that film to a doctor or an ent. Good luck!

TMJ ?: Muscle trismus ? Symptoms sure seem to indicate that. See below link and call your dentist for a follow up. http://en.wikipedia.org/wiki/trismus.

Internal derangement: Disc displacement without reduction. The disc remains trapped in front of the condyle resulting in limited range of motion. Muscles of mastication in co-contraction to protect tmj. Stress of the uncertainty. All this leads to tension type headache. Rec.: see an orofacial pain specialist for disk reduction and amelioration of the condition.