There are many causes of shoulder pain, however one of the most common is injury to the rotator cuff.
The rotator cuff is a group of four muscles which keeps the humerus attached to the shoulder socket.
Activities where there is repetitive motion or recurrent stress can lead to damage of the rotator cuff which in turn can cause pain and weakness when using the arm.
Plain X-rays have limited use in diagnosis since they primarily evaluate the bone. MRI or ultrasound can identify damage to the muscle much better than a X-ray.
Treatment includes rest, anti-inflammatories, physical therapy and sometimes steroid injections. If these treatments don't work, surgery is sometimes needed.
Avoidance of repetitive motions and stretching prior to heavy lifting can help reduce the likelihood or developing a rotator cuff injury.