That same thing happened to my son two years ago. He got this boil at the bottom of his butt. I took him to the ER and they gave him antibiotics. It turned into a staff infection and it literally ate his skin away.

It was freaky looking and the doc put him on 5 different anti-biotic before it healed. Him and his girlfriend were having problems and on the verge of breaking up. So it stands to reason that stress had something to do with it.