What Causes Black Dot On Back For Months?

I had thos black dot on my back for 5 months. I kept squeezing it thinking it is a black. Nothing would come out. Until I recently look at ithad gotton bigger and the skin around it gotten swollen. So I decided to squezze it and this hard blackhead came out with alot of white pus. Alot. And it smelled really bad. I have never had white smelly pus ever come out of a blackhead. What causes this? And should I b worried it might b a major skin infwction? Thnks... Nateli