Lower leg injury

BigJayUK

After my run today after cooling down and stretching.....I got a real bad pain in my lower left leg.....its on the outside and hurts just above the ankle to about halfway up the lower leg. Had ice pack on it for most of the day. It hurts like hell when i start to move..... after about four or five steps it eases off..... but after being stationary for a short time it flares back up again. Any ideas to the causes of it?