Suggest Cure For A Painful Cut Down My Butt Crack

I have a painful red line down my butt crack that's about 3 inches long and starts at the top. It hurts to sit, it hurts to walk, just about the only time it doesnt hurt is when im standing still. This is the third day ive had it and im pretty sure it's not a cut. (oh, and it hurts, not itches) What is this?