Hi Jacobsmom- A sweat test (which is simple and painless) could be given just to rule out CF, and then depending on the results, further testing (such as a dna screen) can be done. If possible, it is best to have it done at an accredited CF center. I have found that alot of doctors are unfamiliar with CF and focus on other "causes", but there are many that have some knowledge of CF. In fact, if your son is diagnosed with CF, you can contact the CF center for names of doctors in your area that are experienced with CF patients and usually work closely with the center.
Some CF'rs have no symtoms while others may be severe from birth so yes, it is quite possible for them to grow well, appear healthy, and still have CF. Being a new mother is hard, you experience many concerns and fears that turn out to nothing--however, if you strongly feel that something is wrong, don't let them "convince" you that you are overreacting. While it could be any numerous other things, if it is CF, it is very important to get him under the care of a physician asap. They will start him on a regime that will provide the best chance of maintaining his "base". While CF is scary, each day that passes is one day closer to a cure. Until that day, keeping them in the best health possible is most important.
I pray for you both that the symtoms are a result of something less serious that can be treated or outgrown, but please keep in mind that if he is diagnosed with CF, while life may be different, it will still go on. Taking one day at a time--enjoying today instead of worrying about tomorrow--worked for me. So, welcome to the forum-if you feel the need to talk-vent-cry-etc., you will always find someone here willing to listen, and if you are a CF mom, will find there are alot of us here and we know how you feel & what you're going through. Let us know what they find out.