What could cause neck pain that is radiating into shoulder?

Several things. Muscle spasm is the first thing that comes to my mind. It could be a disc herniation in the cervical spine. See your pcp as a starting point.
See below. The differential diagnosis for neck and shoulder pain include cervical disc herniation, a tear in the disc, irruption of one of the cervical nerves, pain from the facet joints, and disorders of the shoulder (the pain could originate in the shoulder and radiate to the neck).