There are many reasons why patients experience pain below the shoulder. It is difficult to say exactly what the source of pain might be. Most patients with rotator cuff related pain note that their most significant discomfort occurs on the outside of the arm below the shoulder and radiating down towards the elbow. Shoulder pain does not radiate below the elbow. Bursitis, rotator cuff tendoninosis, tendinitis of the rotator cuff, and other issues within the shoulder can all cause pain within the upper arm. Again, this discomfort should not radiate below your elbow.
There are many many other conditions which will cause pain on the side of the arm, or pain radiating down the entire arm. Another common cause of arm pain. Relates to possible pinched nerve or an issue with your neck. If you experience pain knees areas or any numbness or tingling you should consider seeing your physician or an orthopedic surgeon.