Damage to internal organs will present as pain associated with a particular surface area of the body. Why would something like irritation to the diaphragm, which is between the thoracic and abdominal cavities, feel like pain in the shoulder or neck?

A dish shaped large muscle which moves up and down when there is contraction and expansion of lungs is the diaphragm which is present between the chest cavity and lower abdominal region. The action of the diaphragm is affected by an inflammation occurring below the muscular disc which will affect the process of breathing. The forceful breathing may result in strain and stress in the back muscles of the human body. This in turn causes pain in the shoulder.