The shoulder is a complex joint--this is what enables it to have the most range of motion of any of the joints in the body. There are many different things that could be causing your shoulder pain. First, much of the movement and flexibility of the shoulder is driven by the rotator cuff muscles. A strain, sprain, or tear of a rotator cuff muscle can cause both pain and weakness. In addition, the joint is also held together by a joint capsule, with additional cartilage called the labrum helping to stabilize the interaction between the humerus bone in the arm and the shoulder joint itself. Labral tears or capsular subluxation or stiffness can also be painful and limit movement. The nerves in the neck can sometimes radiate pain to the shoulder area, meaning that pain felt in that part of the body may be 'referred pain' from a problem somewhere else. In addition, some shoulder problems
are often associated with specific patient characteristics: serious swimmers or pitchers often have problems with the rotator cuff or labrum; rotator cuff tears are more common as people age; older women sometimes suffer from 'frozen shoulder' where the joint capsule resists movement. The best way for you to get to the bottom of what is causing your pain is to see your doctor
or an orthopedist for a thorough history as well as physical exam of your shoulder. This should help narrow down the possibilities and determine whether or not you need additional imaging; medications for pain; or physical therapy. Good luck!