Hi I had the same pain when I was in the 3rd trimester, especially if I had eaten a big meal. The doc told me it was the muscle on my ribs stretching to accommodate the baby. Not sure if it is what you're experiencing but it hurt like hell! I always had to make sure I didn't over eat or I'd pay for it later, especially when sitting down. I remember once when I was about 30 weeks DH and I went for dinner and a movie after and I had to kneel on the floor for half the movie I was so uncomfortable LOL.

Hope it goes away soon!