Both tendonitis and bursitis can cause pain, swelling and weakness in the shoulder.
Tendonitis occurs when one or more of the tendons in the shoulder become inflamed or irritated. Tendons attach the body’s muscles to the bones. You may develop shoulder tendonitis from repetitive overuse activity, such as playing tennis, or a more serious injury.
Bursitis develops when the fluid-filled pads in the shoulder (the bursae) become inflamed. The bursae cushions the shoulder joint between the bones, tendons and muscles. Shoulder bursitis is usually caused by wear and tear or an infection.