Difficult to say without evaluation.
Pain and discomfort on one side of the body may suggest a neurologic disease. Obtaining brain imaging, as well as looking at the cervical spine (with an MRI) would be reasonable considerations.
Cardiac causes can also lead to pain traveling down the left arm. An echocardiogram can be perform to exclude major structural disease.
These options can be discussed with your personal physician or neurologist. If the tests remain negative, going to a physiatrist (rehabilitation MD) can be done to look at possible musculoskeletal causes.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
www.kevinmd.com