What is this bump on my back?

Well.. I had this bump on my back and it was small.. (probably the size of a dime) and it was there for a couple years. It was hard and didn't move or anything so i thought nothing of it. Well about two months ago it started to become squishy and so today I pushed on it and allot of this greenish mucus stuff came out so I felt it again to make sure that I got it all of my back and a think blood came out so I went into the bathroom to clean it up and then like watery bloodish stuff came out. I don't know what it is. So can someone help me.? It's on my right shoulder