pimple on butt that got swollen, red and has a yellow head, very painful when sitting

I've had a small flesh like pimple on the bottom of my lower butt cheek for a long time. It never hurt or got irritated til one day I was sweating real bad sitting in a plastic chair outside @ a baby shower. The next day, it was swollen, and has a yellowish head. it's very painful. It looks like something bit me, but I find it odd that a bug would've gotten the chance to bite me in the same spot where my pimple is. Now I don't know what to do to get rid of it. I'm cleaning the spot everyday with acne cleanser and it seems to make it come more to a head, but it won't pop. Please, what can I do, and what might it be