Rotator cuff pain is most commonly caused by an inflamed tendon (tendinitis) or torn tendon. The quality of the pain may range from a dull, aching sensation to sharp pain that moves down the upper arm when reaching overhead or sleeping on the affected side. Interestingly, the intensity of the pain does not necessarily correlate with the degree of the injury. Less commonly, rotator cuff pain may from result from a condition called rotator cuff tendinosis—when the tendons become frayed or worn down as a result of increasing age and overuse.
Source: Verywell Health