I was just watching TV and a yoga expert was saying that a lot of neck and shoulder pain is related to shallow breathing.
I have suffered with neck and shoulder pain and tension for most of my life. She was saying that if you are not using the primary breathing muscles which are in the lower stomach, then you are using the secondary breathing muscles which are the neck and shoulder muscles. And like over using any muscle, this causes pain.
She said that if you place one hand on your abdomen and the other on your chest (this works best if you are laying down), and breath in deeply. The hand on your stomach should move up and down when you breath and the hand on your chest should stay relatively still. This is how you know that you are breathing correctly.