If Your cardiology tests are normal, it can be a myofascial problem in your arm or a pinched nerve somewhere in the cervical spine (upper neck from behind)
Try doing simple Neck isometric exercises with full range left shoulder movements twice a day. Use moist heat fomentation on the arm. You can ise a hot moist towel. (if moist heat on arm helps, its myofascial, if it doesn't help, its a nerve pain
) For more specific and detailed opinion, kindly consult private.