everyone is different and even a non-cfers body is going to lose lung function during preg-remember that.
at 3.5 months i was already down to 55% fev1 from 75%.
you are more than halfway through your preg-the baby, your uterus, and fluids are pushing up on your lungs and diaphragm. i wouldn't worry.
stay active-walk, do 2 pts a day and get back to your pre preg health routine once u deliver. i'm sure you'll be just fine.