Cystic Fibrosis can also have a digestive component to it. My child was born with a bowel obstruction, include malrotation, and some other issues caused by meconium illeus. He's pancreatic insufficient, so he needs to take digestive enzymes to help digest his food -- otherwise he'd have very loose stools and would become malnourished, possibly get another obstruction. He also has more sinus issues.
Your child's sweat test levels must be elevated, therefor the reason for the genetic testing. Normal is 0-40, borderline 40-60, CF 60+. DS passed his sweat test with a normal 32; however, genetic tests showed he did have CF.
So your doctors are being proactive in trying to figure out what's wrong with your child. Hopefully they're doing an ampliefied genetic test -- regular tests only test the most common mutations and there are over 1500 cf mutations...