A build up of pressure inside the ear can occur if the Eustachian tube (the "regulator" tube connecting the inner ear and the nose) does not open to regulate air within the middle ear. Plugged ears cannot only cause pain, they can initiate negative pressure and suction which may result in hearing changes, fluid build up, a feeling of fullness in the ear, and for some - excruciating pain.
Learning how to do the Valsalva maneuver is something every pilot must know how to do to help regulate inner ear pressure with altitude changes. This maneuver as well as yawning or opening and closing the mouth can be all that is needed to help the little Eustachian tube open and regulate the pressure of the middle ear cavity.