What Causes A Hard Lump Below The Left Buttocks?

Hello Doctor Rynne,I am just wondering, I have a hard lump below the skin of my left buttock, the size of a silver dollar, about halfway down. I have no pain. Frankly, I can t see it, and haven t looked. I do have hemorrhoids, but am unsure whether the location would support this diagnosis.I am quite active, am 5 4 and weigh 110.Any insights? Thanks