The pain in your rib cage is most likely your bby sitting there in your ribs. Your uterus is growing and applying pressure on your lungs which makes you have shortness of breath. It's best to sit and stand straight up so your lungs have room to expand when you breath. Ask your Dr and they'll reassure you that it's all apart of being in the last trimester of pregnancy.
I would give your doctor a call and just run it by them and see what they think!