A.C.
It depends on where the pain is. Above the eyes in the forehead it could be sinuses ... especially if you sleep with a fan on (blowing dust around that you're inhaling while you sleep). You can try a decongestant with an anti-infammatory.
It could be stress. It could be tooth issues. It's hard to tell.
If you're not getting any relief from over the counter meds and it continues much longer I'd see a doctor about it.
I had a migraine once that lasted for a week ... talk about NOT fun. One spinal tap to rule out meningitis and some morphine later ... I was good to go :)