Tendonitis is usually caused by an injury , such as a sports injury or an overuse injury when running. Other causes include: Reactive arthritis (formerly called Reiter’s syndrome) – This condition is triggered by an infection in another part of the body like the intestines, genitals or urinary tract. It results in joint pain and swelling. Reactive arthritis typically affects the knees and the joints of the ankles and feet, which may cause tendonitis. Rheumatoid arthritis – This autoimmune and inflammatory disease attacks the immune system and healthy cells in the body by mistake. This can cause inflammation in certain parts of the body, specifically the joints, and may cause tendonitis.