Shoulder pain is a common complaint that has many causes. Since we use our arms for so many activities, shoulder pain may be caused by one or several conditions.
Myofascial Source of Shoulder Pain
The probability of myofascial trigger points generating pain and other symptoms in the
shoulder or arms is very common, but is also the most overlooked and ignored source of
pain in the shoulder.

There are over 20 muscles operating the shoulder and each of them can become a source of pain or muscle/joint dysfunction.

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The National Association for Myofascial Trigger Point Therapy has a very detailed symptom checker with descriptions of all the muscles of the body.

Frozen Shoulder
Is a disabling condition characterized by shoulder pain, stiffness and severe restriction of shoulder and arm movement. Myofascial trigger points in the 4 rotator cuff muscle are the most frequent cause of shoulder pain with restricted range of motion at the shoulder. (infraspinatus, supraspinatus, subscapularis and teres minor ) (click symptom checker)

Thoracic outlet syndrome (TOS)
Is a label used to describe symptoms that are generated due to compression of the brachial nerve plexus and entrapment of subclavian vein or artery
by the pectoralis minor and scalene muscles. Symptoms range from aching pain in arms, disturbance of nerve impulses, swelling, stiffness of finger, tingling, numbness, pinprick and impaired sensation in hands and fingers.

Shoulder Bursitis or Rotator Cuff Tendonitis Bursitis
Is inflammation of a bursa. A bursa is a fluid-filled sac that acts as a buffer or cushion between bone and tendon. Its function is to facilitate friction-free movement for tendon and muscle. Overuse of muscles can lead to tendonitis and consequently to bursitis.

Rotator Cuff Tears
Are tears of one or more of the 4 tendons of the rotator cuff muscles.
A rotator cuff injury can include any type of irritation or damage to the rotator cuff muscles or tendons. (overuse, injury, accidents etc)

Shoulder Instability
Instability is a problem that causes a loose joint. Instability can be caused
by a traumatic injury (dislocation), or may be a developed condition. Calcific tendonitis is a condition of calcium deposits within a tendon -- most commonly within the rotator cuff tendons.

Labral Tear
There are several patterns of a torn labrum (fibrocartilage) and the type of treatment depends on the specific injury.

SLAP lesion
The SLAP lesion is also a type of labral tear. The most common cause is a fall onto an outstretched hand. Biceps Tendon Rupture A biceps tendon rupture occurs when the tendon
of the biceps muscle ruptures near the joint.