Achilles tendonitis is caused by overuse or sudden stresses to your Achilles tendon. Inflammation occurs to the tendon causing pain. The Achilles tendon runs from the bottom of the calf muscle to the heel. The tendon is surrounded in a protective sheath with a layer of fat between the sheath and the tendon. Scars form between the sheath and the tendon when the tendon is injured. Each movement of the tendon pulls the sheath, warming it up and making it softer, which means the pain is reduced with increase exercise. A program of rest and stretches can work to relieve and eliminate pain.