This is pain and swelling of the Achilles tendon, which connects your heel to your calf muscles.
The main symptom is pain and stiffness at the back of your ankle. The pain is usually worse first thing in the morning, or after you have been inactive. Initially you might have pain at the start and finish of your exercise, but this pain goes away when exercising. Without treatment you may get constant pain while exercising. You may also have some swelling and tenderness. If you have sudden pain in your heel or calf, and it becomes swollen, bruised and very painful, you may have completely torn your Achilles tendon. If this happens, seek urgent medical attention.
It is an overuse injury which can be caused by rapid mileage increase, improper footwear and/or tight calf muscles.