Tendonitis

Tendonitis (or more formally tendinitis) is an inflammation of a tendon, the strong stringy tissues that attach muscles to bones that allow muscles to move the bones when the muscle contracts. Through overuse, these tendons become inflammed, painful and unable to pull through their full range of movement. Any tendon can become inflammed, but tendons that are responsible for large ranges of movement are the most likely to do so, such as those in the heel, knee, shoulder and elbow. It is very common in athletes, although it can affect anyone.
Tendonitis can be extremely painful, and can often be mistaken for a sprain or strain. It often comes on with no trauma, and even after the tendon has rested for a time (such as overnight). The muscles on either side of the tendon often become very sensitive to touch. The pain can persist even when the affected body part is at rest.
Tendonitis is rarely serious, although the pain can interfere with sleep patterns. The pain and inflammation will go away with rest and use of the affected body part should be avoided. Non-steroid anti-inflammatories, such as Advil, should be taken to relieve the symptoms.
Tendinitis at Wikipedia