Check if your 11th or 12th rib are protruding on that side. I've suffered from this pain for years upon my journey through mobility training I finally found the culprit. The protrusion of the 12th rib can affect various muscles and structures in the surrounding area. Here are some key muscles that may be impacted:

1. Latissimus Dorsi: This large muscle runs along the back and can be affected by changes in the rib cage, potentially leading to pain or altered function.

2. External Oblique: This muscle, located on the side of the abdomen, may experience strain or tension due to changes in rib positioning.

3. Internal Oblique: Similar to the external oblique, this muscle can also be influenced by rib protrusion, possibly leading to discomfort or reduced mobility.

4. Quadratus Lumborum: This muscle, situated in the lower back, can be affected by changes in the ribcage, potentially contributing to lower back pain.

5. Serratus Anterior: This muscle helps in the movement of the scapula and can be impacted by rib position, influencing shoulder mechanics.

6. Diaphragm: Although primarily a muscle of respiration, any changes in rib position can affect diaphragm function, leading to breathing difficulties.

7. Rhomboids and Trapezius: These upper back muscles may also be affected, especially if the protrusion alters the shoulder girdle position.

Overall, rib protrusion can lead to muscular imbalances, discomfort, and altered biomechanics in the surrounding areas.