What Causes Bump On Stomach Oozing Pus When Pricked?

I have a bump on my stomach that has been there for a few years. I squeezed it and a head formed, but nothing came out. I poke it with a needle, and a thick cheesy white substance came out, and a lot of it. Today it is red and swollen and hurts...I squeezed it again, and a very large amount of softer white stuff came out. What is this?