The doctor is probably basing his or her diagnosis on your symptoms and any findings from the physical exam. Clinical cervical spine instability (CCSI) is a fairly common problem that doesn't always show up on standard X-rays.
Most patients report clicking, popping, or a clunking sensation with neck motion. Neck pain is a key feature. Headache pain is less predictable. Abnormal neck motion is more common than hypermobility (too much motion).
All structures around the head and neck work to hold the spine stable. The goal is to allow normal control of posture and movement without pain or dysfunction. With CCSI the spine is unstable under normal loads, forces, or movements.
There may be increased shearing forces at the joints. Pressure on the nerves can cause shoulder and/or arm pain. Muscle weakness can also occur with pressure on spinal nerves.