What Causes A Boil On Hip?

i have a some kind of boil on my hip (exactly from the place where potty comes out). I can't sit properly and its paining a lot. Its been 4 days now. I doubt whether it is baal tod or something else.... today i;ve started taking ampicilin. If its baal tod then i really dont want to get it done by ripping it. It also seems like there's some PUS inside......Please advice...kapil