What is upper left arm pain in a 22 year old female?

Shoulder pain in : The upper arm is most commonly referred pain from a rotator cuff issue. The most likely diagnosis is rotator cuff tendonitis associated with impingement. If you have weakness or significant night pain you might also be worried about a rotator cuff tear, albeit rare in those under the age of 50. Treatment initially is nsaids, ice and gentle rom. If not getting better see your doc.