I saw your post on PgAL and I wanted to tell you I am so sorry you have to go through this - I can totally relate. We found out two weeks ago that our LO does have CF (DNA test doesn't lie, unfortunately). I wanted to let you know that I have done a SH!T ton of research in the last two weeks and met with countless doctors regarding what to expect. I am here for you if you have any questions. If you click on the CF FAQs in my siggy it will take you to the CFF website where you can find a lot of basic information (but I assume you have probably already been there).
A few things - false positives are REALLY common for sweat tests - mainly because many newborns don't sweat alot (or on demand for the test). Did they tell you what her number was - it could be borderline and that is why they are doing another test.
There is no way to 100% confirm the diagnosis of CF without a genetic test. I would actually demand that they do one on her to eliminate or confirm the results. Sweat tests can produce false negatives too, so IMO it's best to know for sure.
I know how terrified you are - it's the worst feeling in the world. If you need anything at all, please let me know.