There can be several causes for a localized swelling in the buttocks, such as epidermal inclusion cyst, infected lipoma, abscess
, boil/furuncle or fibroma. It would be difficult to diagnose without an examination.
An abscess is a localized collection of pus walled off from the rest of the body. It contains pus, which consists of inflammatory cells and decayed tissues. Most abscesses causes local pain and tenderness.