Mild pain on left side neck, shoulder, & arm and back. occasionally comes and goes. what can be the cause?

Cervical radiculitis: Your symptoms can be due to a wide variety of conditions, and for this reason only a thorough history and physical exam can provide you with an accurate diagnosis. However, people with a cervical radiculitis (irritated nerve root in the neck region) often report symptoms as you describe. I would recommend you see a physician such as a physiatrist for an evaluation.