The tendon in the back of your foot is called the Achilles tendon. It is the strongest of the tendons in your body because it has to bear your weight and the force of your walking, running, jumping, and so on.
Because the tendon is under so much stress, it can be damaged through over use or with trauma. While no recommendations can be made without an examination of your foot to tell what the problem is, there are some issues you may look at to see if they prevent pain.
First, are you a regular athlete or do you just exercise occasionally? If you exercise only once in a while, this could be why your tendon is hurting. On the other hand, if you participate in regular exercise, your tendon may be suffering from overuse.
Do your shoes fit properly and are they the right shoe for your activity? It is amazing how many people aren’t wearing the correct shoes for their particular activity. Be sure they’re the right shoes and that they fit well. As well, make sure they’re not too old.
If your tendon pain continues, you should be examined in case you could be making things worse.