What would cause a days-long feeling of pressure in the head, as if you've suddenly ascended to a high altitude and your ears have not yet popped? it's not painful, but the sensation is unpleasant and persistent?

Fullness Head: Exposure to carbon mono oxide, retention of Co2, high blood pressure, hypervitaminosis A, significant wax in both ears, changing time zones or altitude rapidly , increased viscosity of blood due to cells or proteins are some of the reasons for feeling fullness in head. See your doctor.

Head pressure: Usually this happens with congestion secondary to allergies usually. You can try antihistamines and see if the symptoms imptove. If persistent please see a physician for examination.