I woke up 2 days ago with sharp pain behind my knee when I put pressure on the ball of my foot. The exact location of the pain is on my right leg, behind the knee, on the upper right area of my calf muscle. I was playing DanceDanceRevolution in the days before, but no noticeable pain occurred then, and that was the only physical activity I really had in the days before I woke up with this pain. Anyone have any ideas what this could be?