This symptom may be due to scoliosis (curvature of the spine).
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.