Tendonitis can flare up due to various reasons. Overuse or repetitive motions are the most common causes of tendonitis flare-ups. Activities that include repetitive actions such as typing, painting, playing sports or musical instruments can put excessive strain on the tendon and cause it to become aggravated. Other factors that can contribute to tendonitis flare-ups include:   1. Aging: as we age, the tendons become less flexible and more prone to injury. 2. Poor posture: poor posture can put extra strain on the tendons and cause them to become aggravated. 3. Improper technique: improper technique while performing physical activities, sports, or exercises can also cause tendonitis. 4. Poor nutrition: missing essential nutrients like Vitamin C, E, and Zinc can lead to weakened tendons, making them more susceptible to injury. 5. Medical conditions: rheumatoid arthritis, gout, and diabetes can also lead to developing tendonitis.   If you are experiencing tendonitis flare-ups, it is important to identify the reasons why and take steps to address it. Your healthcare provider may recommend rest, physical therapy, or other treatments to help reduce inflammation and promote healing.