What might be causing pain in my shoulder/neck area that radiates down the side and back of my arm. i get a strong aching pain when i move my arm.

See your doctor: Pain that severe needs to be worked up by your doctor in person. He/she will listen to you, perform an examination and possibly order some tests to see what's wrong with you.

Many Disorders : You need examination of your neck, shoulder, & arm. Since pain is aroused by shoulder movement, this suggests neural pinching in the shoulder. Thoracic outlet syndrome is a consideration. So is a disc protrusion in neck spine. An osteopath or physiatrist can examine, order tests, & define. Mri probably needed. Ultrasound can help decide. Physical therapy if spinal disc problem not found.