Achilles Tendonitis is a condition affecting the Achilles tendon which is the large tendon that joins your calf muscles to the back of the heel. (It is the one that women like to make tight to look good in high heels!) Tendonitis is a condition where the tendon is inflamed. The ‘itis’ means inflamed. When the tendon is inflamed it can be very painful, particularly after you have been resting for a while, as in sleeping. There are many different causes of Achilles tendonitis.