Probably you developed a haematoma
as a result of trauma you sustained which is gradually dissolving. The swelling is a result of inflammation of the soft tissue
and will gradually subside. You may take a pain killer and apply some local analgesic
like volini gel. Do not over exert. I would like to suggest that get a x ray of the area done to rule out any bony injury
in which case you may have to consult a orthopedician.