Pain in left side of back of neck when sitting, radiating to head and shoulder, occasional tingling. Xray shows no bone issue, but pain can be severe.

See a Spine Surgeon. You may need further evaluation if symptoms persisting. You may need MRI of your cervical spine -- which may pick up other problems not seen on xray which may then identify the cause of your symptoms.
Many causes. You may need a cat scan or MRI to fully diagnose why you mhave this. Get examined by neurologist.