What Causes Large Bump On Butt Cheek Which Is Reddish And Shrinks After Bursting?

I have a large (half dollar x maybe half inch thick) bump in the middle of my right butt cheek. It is kinda of hard has not head, is reddish with some darker colors( like bruising). It is very painful to sit on, and seems to be growing in size...When it first appeared I had got it to rupture and it was as big almost like a boil but it was a lot of discolored blood and clear fluid. And at that time had no head either... It seems as though it might be an infection the way it has spread since then but after popping it it shrunk in size for a while and stopped hurting till recently... What could it be and what can I do about it?