Shoulder tendonitis is inflammation or irritation of a tendon that causes pain and tenderness in the shoulder, which may occur with repeated shoulder impingement (rotator cuff) injury. Tendonitis can occur in any tendon in the body, but tends to be most common in shoulders, elbows, wrists, and heels. Tendonitis is also sometimes called tendinitis. The rotator cuff tendon becomes swollen and inflamed over time. Shoulder tendonitis is classified as either: Acute Shoulder Tendonitis: Brought on suddenly by excessive overhead activities. Chronic Shoulder Tendonitis: Typically caused by a degenerative disease, such as arthritis, or long-term repetitive wear and tear.