Possibly pinched nerve in upper left shoulder area. radiates up neck and down left arm. very painfully. only relief when i lay flat on back.

See details: This issue needs to be evaluated and treated. Yes it could be related to a pinched nerve.

Radiculopathy: You should see a doctor. It sounds like you have a radiculopathy, in which a cervical nerve is being compressed, possible by a cervical disc.

I PAIN: You need ct scan/mri of cervical spine to see disc, vertebra, nerve compression, osteoarthritis, fasciae and muscles. The appropriate management depends on correct diagnosis. Please see your doctor. Thanks.