Like Heather said, sometimes this is normal behavior, but sometimes, with some kids, it is an early sign of infection. It might also be reflux, which has a high incidence in CFers, too. These are all issues you can discuss with your clinic and see what they think.
We can often predict a coming infection or sinus trouble with my youngest because the appetite drops. Many CFers get sinus issues, and when it is the change of seasons, that can mean lots of extra goo draining into the stomach. Our youngest went through a period where we could not get her to eat in the mornings; her stomach was upset because of sinuses. She had sinus surgery a few months ago, and her appetite is much better.