What Causes Bump On Upper Bsck?

Hello, my mother has a bump on her upper back that looks like a blackhead. We popped it and a lot of smelly puss came out. It hurts her and finds comfort when we push the puss out, which can be up to four times a day. She has had it for a couple of years, but before the blackhead would come out she hadn t been able to remove the blackhead and that s when her pain started and she noticed the ball get bigger. She didn t go to her PCP, but they just did exactly what we were doing at home and gave her antibiotics. She has not found any comfort and would like to know if this would be a dermilogical problem. Thank you for your help -diana