What causes sharp pain at the back of my ankle? the pain comes and goes and when i have the pain it hurts so bad i can't walk on it. its not swollen

Achilles tendinitis : The most likely cause for this type of pain is what is called achilles tendinitis. This is a small tear of the achilles' tendon or inflammation around the tendon. Some simple ways to prevent this is to do some stretching exercises. You could also benefit from a custom orthotic. I would recommend seeing your foot and ankle specialist such as a podiatrist or orthopedist to prevent further injury.