I think it's just everything moving and adjusting. Some of your organs are getting squished up more into your rib cage area, so your ribs get forced apart a bit (like when you take huge breaths) and the intercostal muscles (btn each rib) can get stretched and sore. Your diaphragm can get put under pressure too, causing more ouchiness. I'm now at the point where DS's bum is under one boob and feet under the other, so all the wriggles and stretches he does makes my ribs very sore. Pushing him back 'in' helps as does a warm wheat bag on my back ribs.

Just remembered that SIL had really sharp pains in her lung area and had to be checked out as she has a bad clotting disorder. They did all sorts of tests, including some sort of radioactive check on oxygen movement and concluded that it was just bubs kicking and restricting her breathing.