Considering your symptoms of jaw tenderness and pain on chewing, your symptoms could be due to temporomandibular (TMJ) joint arthritis. This is the jaw joint and inflammation of this joint can cause pain on opening the mouth and tenderness on palpation behind the ear. The pain could also be due to an enlarged and inflamed lymph node or salivary gland enlargement. Please consult a dentist for assessment. He may ask for blood tests and an X ray. In the meantime take OTC acetaminophen for the pain.