Sinuses are hollow pockets in the skull that are connected to the nasal passages. When allergies trigger swelling in the mucous membranes, the inflamed tissue can block off the sinuses. The sinuses can’t drain, trapping mucus and air inside. That leads to pain and pressure.
To prevent rebound congestion, use over-the-counter decongestant nasal sprays for no more than five days in a row, with as few doses as possible each day. Prescription nasal sprays containing steroids don't cause this rebound effect, so they can be used on a daily basis for years.