It sounds like you may have a rib out of place.  You may also have a muscle in that area that's irritated - maybe your rhomboids, lat, or intercostal muscles.  A gentle stretch where you reach up over your head to the side or one where you reach out and hold something while leaning back may help.

It will eventually settle out and shift into place, if it doesn't see you chiropractor.  Heat and/or a muscle relaxing gel like biofreeze may speed up the process.