Shoulder Pain can be caused by many conditions.The pain can be the result of a traumatic injury, though it is more common to have shoulder pain due to repetitive stress, improper posture, arthritis, or chronic inflammation.You may be told that you have a rotator cuff injury.The rotator cuff is the tendons of the four main muscles in the shoulder.People dealing with shoulder pain often have trouble sleeping on their affected side.And injury in the shoulder can also lead to frozen shoulder, which severely limits range of motion.This makes reaching up or behind difficult and painful.