C.M.
I had the same thing with my daughter when I was pregnant with her. It seemed to start about the same time, 7 months. (She was a much bigger baby than my son) I can't rememeber the medical term, but basically, according to my Ob, the tendons and cartilidge between your ribs are being stretched. I felt as if she was trying to climb up under them, and it helped to straighten my back and "push" her back down.
It was particularly bad if I was sitting in my office chair for too long. Also, while sleeping, I got a wedge pillow which helped support the belly so the weight wouldn't pull on my ribs so much.
Iif you work, take lots of breaks from your desk and just walk around. Check your posture and sit up straight. I did yoga during my pregnancy with her and I always felt better after a relaxing workout.
The good news is, it was immediately gone after birth! Good luck!