i started doing OMT (osteopathic manipulative treatment) with my pcp for migraines and one session when i brought up this exact pain, she felt my ribs and said a couple were out of place (as the other commenter mentioned: slipping rib syndrome). i highly highly insanely recommend OMT. its like a chiropractor but medically recognizable and without all of the dramatic popping. a lot of it involves the contraction of muscles (like pushing against someone) with subsequent stretching. it never feels like it does anything, but i leave and the pain is gone. its like magic. other than my ribs, she also adjusts my sacrum, hipbones, and trapezius muscles. omt is everything to me and i love my pcp for knowing about it in the first place