What is Achilles Tendonitis? Achilles Tendonitis occurs when the Achilles Tendon in the back of your ankle becomes irritated and inflamed. This is usually due to overuse. There are two types of Achilles Tendonitis, affecting two different areas of the tendon. These are: Insertional Achilles Tendonitis Insertional Achilles Tendonitis affects the tendon at the insertion ...