What Causes Persistent And Painful Sore?

I have a pore on back that is continuously sore. It looks like a blackhead, also it contains puss. If I apply pressure I am able to excrete some of the puss. I have had this condition for a few months and the pore continues to contain puss and cause pain. The pain originates in the back and also causes discomfort in the right shoulder. Just wondering what it could be.