The most common cause of shoulder pain when the arm is in a certain position is called impingement. This can occur in the front or back of the shoulder when something is getting pinched. Most often, a tendon or bursa (protective sac) gets pushed against a bone or ligament.
Based on what you describe, there are other possible causes that could be more serious, such as a tumor. Although this is uncommon, a medical doctor must make the diagnosis. Impingement is commonly treated with physical therapy, though sometimes surgery is required. Some tumors respond to radiation; others must be removed surgically.
Early diagnosis can help prevent the problem from getting worse. Small tears in the shoulder can become large tears without treatment. Call your doctor today for an evaluation.