Shoulder tendonitis in short is an inflammation injury to the tendons of the rotator cuff muscles. However, inflammation may not always be present in shoulder injuries therefore those injuries can be referred to as rotator cuff tendinopathies.
The most common cause of tendonitis is repeated micro trauma to the RC tendons. As previously discussed, RC impingement should not occur during NORMAL shoulder function. However when this repeated impinging movement occurs, your tendons can become inflamed. It is also possible to have shoulder bursitis in conjunction with RC tendonitis.
Unfortunately, many other injuries can accompany RC tendonitis as mentioned above regarding bursitis, biceps tendonitis, calcific tendonitis which can eventually lead to rotator cuff tears.
GOOD NEWS! Most RC tendonitis is reversible and can be successfully treated! Great, but how? Consult your physical therapist who would assess your shoulder biomechanics and utilize special testing for diagnostic purposes to correct your shoulder movements and prevent future impingement episodes or subsequent RC tendonitis. There are a number of phases in rehabilitation of your shoulder injury.
(Disclaimer: Guidelines followed by John Miller from Physioworks)