K.O.
It could be a clogged milk duct. If your fever returns go to the doctor; clogged milk ducts can make you very sick. I know they hurt like heck; it's happened to me. Keep massaging it, and you can also try using a warm compress. What worked for me is nursing, but it doesn't sound like you're doing that so I'm not sure what else to suggest. But really.... go to the doctor if this doesn't clear up soon!