Weird burning in my leg?

I was sitting in my chair (a lawn chair improvised as a desk chair) watching a video when I had this extremely painful burning on the back of my left leg, just above the knee. It was as if the edge of my chair had suddenly became burning hot, because the only part that hurt was the part touching the edge of the chair. The pain faded away over the course of about twenty minutes, but it was still painful to the touch for the rest of the night. There were no marks that I could see. Does anyone have any idea what could have caused it?