Varied. Many. Could be tooth problem, jaw joint problem, muscles in pain around TMJ. Bruxism, parotid gland problem.. I would start with an oral surgeon to diagnose/make recommendations.
Parotid tumor. If this is in front of your ear. It could be a tumor of the parotid gland. See your local ent.
See an ENT. The major cause of swollen parotid glands is a virus that causes mumps. The cause of the swelling must be identified and visit to an ENT is warranted.
Acute infections. If the issue is acute could be infection either in ear or gums, parotitis, tumor if this persists.