I Have A Sore Spot Inside Of My Butt Cheek.

I have a sore spot inside of my butt cheek. It started out was like a pimple so I mashed it in the shower and thick infection came out and was gone. The next day, it became the size of a nickel and now it is the size of a half dollar and now getting close the the anus opening. It hurts to sit and even use bathroom. Any suggestions what it is and how to get rid of this?