Sudden swelling of the ear flap can be caused by an abscess or hematoma. Abscesses frequently occur after dog fights. A hematoma is an accumulation of blood beneath the skin. One cause of hematoma is violent head shaking and scratching at the ear. The area will feel warm and slightly soft to the touch. It may be painful. Look for an underlying itchy ear disorder.
Treatment: Blood must be released from a hematoma to prevent scarring and ear deformity. Removing it with a needle and syringe (something your veterinarian must do) is effective in about 20 percent of cases. If serum accumulates in the drained blood pocket, treatment involves removing a window of skin to provide open and continuous drainage. Sutures are then made through both sides of the ear to pull the skin down and eliminate the pocket.
Sometimes a drain will be installed under the skin of the ear to serve this same purpose. If the blood is not removed, the cartilage will curl down and deform the ear as the clot retracts inside.
The treatment of abscess is discussed in Cellulitis and Skin Abscess.