You should definately have another sweat done if they did not get a quanity sufficient for testing. It is reccomended by the CF foundation that the sweat test is performed twice to confirm, especially if your do not have sufficient quanity for testing.
In additon to having another sweat test, you could have a genotype done. Its a simple blood test that usually takes a couple of weeks to recieve results. There are known mutations that do not produce an abnormal sweat test result, so it is possible to have a negative sweat test and still have CF.
Hopefully you do not have CF, but if you do, the sooner you can start being treated, the better you will feel.
Hope this helps..