I have pain pressure in back of head and pressure in face. pain in head has been for three weeks face pressure just started today. what could this be?

Sinus pressure?: This could be just simple sinus pressure. A quick way to test this is to try an over the counter decongestant pill or nasal spray. If you don't have much improvement after several days, you should see your doctor for a good ENT exam.

Could be TMJD: Some people call it tmj. It may be tmjd, which is temporal mandibular joint disease. You are probably clenching or bruxing. Seek dental attention , but the right dentist, as all do not have the same level of training! can lead to more health issues, can get worse progressively.

Sinus or allergy ?: First question i would ask is do you have history of allergies? Or is their a possible sinus issue? Do you have sensitivity to light? And, what kind of facial pain?

Symptoms vague: Vague symptoms. I'd start with an ENT doc. If negative See a TMJ expert. 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.