I got sick of being treated like a lunatic mom who saw something wrong in everything when my son was "fine" according to his then-ped. Get a second opinion. Usually pediatricians work with fellows so unless it's a single-dr office (which sometimes spells entrepreneurship but mostly spells "OTHER PEOPLE CANT STAND WORKING WITH ME BC MY EGO'S TOO BIG!"), you should easily be able to request a second opinion. Don't worry about hurting the dr's feelings....they usually welcome the chance to be proven correct. If that is a concern though, or if you want an expressly unbiased opinion, seek out a different (well-recommended) pediatrician. I had to do the same with my son as an infant and was subsequently referred to a pediatric gastrointerologist when he was 6 months - found out he had a hernia, reflux, colic, and was allergic to EVERYTHING (had to be put on a completely synthetic formula - alimentum). We did some abdominal massage to get the hernia down so his tissue could grow over it and as soon as we to the formula, he was a completely different baby.
Get a second opinion....ask friends for a good ped, or do some research on the internet! And remember - dr's are great but they don't know everything. What they think may be a fretful overly concerned mother could be an actual medical problem they hadn't considered. *OR* it could be nothing much that she will grow out of.
GOOD LUCK! Our prayers are with you :)