If the lump came up really quickly, it is possible it might be an abscess
(?) Even more likely if it is very sensitive or painful, or looks inflamed. If it's a very soft, round lump, it might be a lipoma, which is benign, but if it grows too big, and affects the nerves or muscle-function, may need to be taken off, (a lipoma is made of fatty tissue) If it is firm, but smooth and very rounded, there is a possiblity it could be a fluid-filled cyst. That could be drained.