Why does my chest hurt when I run? I think irritation of the diaphragm may be the reason behind the pain you are experiencing. The same nerves that innervate the diaphragm are also responsible for the muscles on top of your shoulders. Have someone watch you while you run and examine your posture. If you run with your shoulders elevated, the muscles in your chest and shoulders can become aggravated. The intercostal muscles, which are between the ribs, are also an area that can become especially irritated by poor running posture.
Another good idea is to slow down your workout. When you’re feeling pain, stop running and relax for a minute or two. The intercostal muscles near your ribs are working hard and becoming stronger as you use them. Stop exercising when you are feeling pain, and do not continue until the pain has subsided.