This condition called "Brachial Neuritis", Feel Good Knees for Fast Pain Relief Review or "Brachial Neuralgia", is due to the involvement of the group of nerves exiting from the spinal vertebrae, called the "Brachial Plexus". The pain can be very intense on any movement of the arm and any pressure along the distribution of the nerve pathway. Pains that occur over the shoulder blade (scapula) area can be due to the large muscle (trapezius) and underlying smaller muscles that have become spastic. The spasticity of the muscles can be brought on by overuse or trauma to the muscle.
The muscle will develop a nodule or nodules that will in turn cause trigger points that often send radiating pains to sites far removed from the trigger points.
Pains that occur under the shoulder blade may signal a warning that the pain is not in the shoulder itself, but can be related to a pain from one of the organs in the body. The shoulder where the pain occurs can give the clue which of the organs are affected and causing the radiating pain into the shoulder.