I Have Had A Knot Near My Lower Rear Cheek

i have had a knot near my lower rear cheek that is a purpley blue in center and red and shiney around it the knot will swell to about grape size or larger and when i apply pressure it ruptures with thick bloody pus then the swelling goes down but after a few hours swells back up its very painful and has been there for a month or more please what could it be