A couple years ago I suffered an injury that was literally a pain in the butt. My doctor was pretty certain it was piriformis syndrome. It's caused by compression or entrapment of the sciatic nerve by the piriformis muscle where the nerve passes through the gluteal muscles (buttocks). The pain can be a constant ache or sharp pain in the hip or radiating down the leg or into the lower back. It's also been called sciatica. Fortunately it can usually be treated and prevented with some simple stretches.