What Causes Fluid Filled Painful Boil Near Sacrum?

Oh, ok....obviously embarrassing but curious what it is and if a major concern....just above or at the top of the rear end at the top of what some would joke as the crack, I have something like a boil or maybe like a large pimple, haven t been able to look at it etc....after a few days of pain where it s difficult to sit, stand or even lay down it will sort of pop with a large discharge then it shrinks. When it does burst, I feel better almost immediately......any ideas???