Sinuses are air-filled spaces located in your forehead, cheekbones, and behind the bridge of your nose which drain through narrow channels into the nose. When the linings of the channels that connects the sinuses to the nose become inflamed, they impair the ability of the sinuses to drain normally. Pressure may begin to build up within the blocked sinus.
The swelling and inflammation then back up into the sinuses with increased mucus and fluid secretion. Pressure can also develop at contact points between two structures in the nose and sinuses that swell against each other. All of these factors can combine to create the pain of a sinus headache.