What is this odor from a pimple on my back??

I have this bump on my back (like a pimple). If I squish it, this white substance comes out and it smells TERRIBLE. After that it will reappear in a week or so and I squish it again, same thing... week after week, year after year, basically all my adult life I have had this thing. It's nearly invisible by looking, it's just a bump that holds this stinky goo. What could it be?