lump or bump underneath skin

For about three years I've had  a  hard lump/bump underneath my skin 2 inches directly beneath my belly button.  The lump has a black dot on the center.  Previously I used to squeeze hard it from time to time and a black/brownish stringy pus used to shoot out.  After squeezing as much as I could, the lump would decrease in size and feel a little bit softer.  

Last night, I noticed the lump felt more tender and after squeezing (this time i didn't have to squeeze very hard), instead of the normal stringy pus, it was small chunks of pus that had a little bit of blood.

What could this thing be?