What Causes Red Bumps With Puss On The Leg And Butt?

I have red, raised puss filled bumps on my upper leg and butt. Just noticed one on my chest and one on my hip. They are surrounded by a red rash about the size of a silver dollar. Then they leave purple scars with an initial purple circle with a faded purple circle the size of a nickel. This happens over the course of 7-10 days and has been going on for a few months. They hurt!