Hi, I've had this tiny little lump behind my knee, maybe half the size of a dime, for quite a few years. over the last few days however, it has ballooned up to about half the size of a ping pong ball. Just a little while ago, it seemed to pop, without any pain or anything, just found blood leaking down the back of my leg, along with a foul smelling pus. Any idea what this is?