Piriformis Syndrome?

After a “heavy” bout of stretching (lower back/hips/hamstrings), I felt fine BUT when I went to sleep that evening I suddenly developed a sharp pain in the center of my left buttocks and it shot down the outside of my leg. There was no position I could lay in to ease the pain. the wierd thing is that during the day i feel no pain or stiffness whatsoever. But for a week now, evry time I lay horizontally, the pain develops to the point where it is impossible to sleep. Each morning I get up, and within an hour the pain is gone. I can still swim and even run with no pain during or after the exercise. Any ideas?