I agree with welshwitch, tiffanymarie...
I had always been thin -- probably a combination of pancreatic insufficiency (not making the enzyme that breaks down fat - a MAJOR component of our diet), malabsorption, and the raging lung infections we tend to get... I think the lung infections have a major impact -- but all of them together seem to form a perfect storm... I had a double lung transplant 14+ years ago and the raging lung infections are gone. And, I'm now struggling to maintain a good BMI / weight because I find it VERY easy to gain weight for the first time in my life!
So -- if you managed to get by during early years without many infections... And if your absorption is OK, and if your pancreatic activity in producing that digestive enzyme is reasonable -- it's very possible that you might not be thin or malnourished...
I agree with you about the leap of faith that all/most CFers have historically been thin... I recall years ago, when any group of us were together at CF walks and whatnot -- it was always easy to tell just by outward appearance who was CF and who was not... That's not the case so much anymore. There have been improvements in getting nutrition into us -- but also, we're seeing many more of us who are impacted by different symptoms and mutations...
Sorry you're in need of some type of diagnosis -- but I hope they come up with the right diagnosis soon, so that you can begin appropriate treatments... Love, Steve