A common cause of shoulder pain is soreness of the tendon (a cord that attaches a muscle to a bone) in the rotator cuff. This is the part of the shoulder that helps circular motion. Another common cause is soreness of the subacromial bursa (a sac of fluid under the highest part of the shoulder). You might experience soreness after painting, lifting items, or playing a sport—anything that requires you to lift your arms, especially overhead. Or you may not remember any specific injury, but you still feel pain in your shoulder.