By Nancy Snyderman, M.D.:

"There is something called a pressure headache, which is due to underventilation of one or more sinuses. The sinuses are essentially air pockets in the bones of your face. Air gets into the sinuses via bony passages that are quite narrow. The bone of these passages is lined with mucosa, the same tissue that lines the sinuses and nasal cavities. If this mucosa is swollen (for example, as a result of recent infection or due to allergy), the passage may swell shut. The result is an underventilated sinus. Air cannot get into the sinus, and mucus produced within the sinus cannot get out.

The air trapped within the sinus dissolves gradually into the mucosa lining the sinus. The air pressure within the sinus gradually falls. The net result is lower air pressure in the sinus than in the outside world. You perceive this as facial pain and pressure."

http://www.ivillage.com/treating-sinus-pressure-headaches/4-...