there are many possibilites, from ribs to muscles to internal organ refer pain. you need to see a doc and get examined if pain persist.

I have had that off and on for years, but not any longer. My husband does therapeutic bodywork, and for me it was a stuck rib due to old spinal subluxations, which I've been gradually working out through limited good chiropractic and bodywork. Sometimes it's ligaments, fascia, etc. but my gut feeling when you mention that is structural. A really good bodyworker (and I don't mean the average massage therapist), maybe someone who does Rolfing, Zero Balancing, Visceral manipulation, myofascial release, etc. might be able to help.

that is just my 2 cents, I don't know if it's OK to post that here, it just sounded so familiar. If you can't get a medical answer, the structural alignment/bodywork idea might be worth exploring.