The Achilles tendon, or “heel cord”, connects your calf muscle to the heel. Overuse can inflame the tendon, or even tear or rupture it, causing a lot of pain.
Sudden increases in intensity of exercise can be a cause. It is more common in men, and as you get older. Running a lot in the soft sand of the beach has caused problems in many of my friends. Achilles tendinitis can be caused by certain antibiotics, such as fluoroquinolones.
Varying your type of exercise, or cross training, distributes the strain you put on your body and decreases the likelihood of Achilles tendinitis. Swimming comes to mind with any disturbance of your lower extremities. The pool in which I swim three days a week is disparagingly called the “injured reserve pool” in my community Stretching before exercising is always a good idea. Pick a routine and stick with it, particularly as you get older. If you have flat feet, getting a slight lift for the heel may help to take tension off the Heel cord, and orthotics may help. Getting expert advice is always advisable.
Prevention is always far better than treatment.
If a short period of rest doesn’t get rid of the pain, check with your Doctor, who might examine the tendon, take x-rays, or order ultrasounds to check the extent of the difficulty.