Hi there
The muscle involved is the anterior serratus, the lower section of the muscle. On an energetic level this muscle is related to the lung, and could indicate a problem with the lower lobe of the lung. On the left the lung has 2 lobes (in most of us) on the right 3 lobes.
We test the muscle by having the arm extended horizontal to the front and applying slight pressure on the wrist. If the muscle 'gives' it indicates a weakness in the muscle and the lower lobe of the lung. Reasons could be plenty. Are you a smoker or live with a smoker?
Leaning to the left would shorten the muscle somewhat and alleviate pain so I think I am talking about the right muscle. The anterior serratus runs from the outer surface of the upper 9 ribs to the inner surface of the shoulder blade. It actually draws the shoulderblade forward and elevates the ribs.
It is often involved in breathing problems like emphysema. One of my books (Touch for health, John Thie D.C) tells me foods rich in Vitamin C and increased water intake are indicated.
When these muscles are over tight because of a lack of Vit C and water, they tear easy. It makes you feel like you have to breathe shallow.
2000 mg of Vitamin C and about 3 liters of unchlorinated water would be needed.
Massaging the points between the 3rd and 4th ribs, next to the breastbone will be beneficial but painful. Do this about 8 times a day for 30 seconds at a time. These are neuro-lymphatic points and will 'reset' the energy to the muscles.
Good luck
Kinetico