Hello~That particular rib may be out of alignment, a simple chiropractic adjustment would help to get it back where it belongs, I know as this has happened to me. You also may have costochondritis, this is a very painful condition of the ribs, and it possibly could cause the cartridge around the rib to swell, thus causing it to protrude more.
This symptom may be due to scoliosis (curvature of the spine).