@txmom33 @julie1073 , I wrote this post 5 years ago and here's what we learned after 2 yrs of misery. It was C. difficile (see the chat boards here about this diagnosis, as well as the listing on the main Mayo site), but we had gotten two false negatives on the first tests, so we went many months with bad information. In the meantime she ended up on feeding tubes. If, after researching this illness, you think you child might need to be tested for that, ask for the Cytotoxin test (a.k.a, the gold standard test for C. diff.). That one is most accurate and I think the only one they use now, but that's the best one. The antibiotics to treat it were no fun and she got reinfected 3 times because those superbugs would not let go. We cured her instantly with a fecal transplant. It's a miracle cure for persistent C. diff.
C. diff. is bad because it builds up toxins in the gut that can eventually affect the sympathetic nervous system. This happened to my daughter and she got a secondary condition of Postural Orthostatic Tachycardia Syndrome, or POTS. This is also known as autonomic nervous system dysfunction. [Ironically, the doctor who suggested it might be a problem never explained to me what it was, so we went another year before we could get in to Mayo (yes, it took a YEAR! I understand they have shorter wait times now) and that's exactly what it was.] You can look up POTS here too, but symptoms are racing heart, nausea, overheating, flushed face, fatigue, fainting.
We also found out at Mayo that she had a fructose intolerance. Fructose is in most fruit and many legumes and some veggies, so we had to learn what to eat. Any sort of simple sugar malabsorption or intolerance can cause tummy/gut trouble and vomiting, so ask your doctor about that.
I know how stressful this is and how much you agonize watching your kid suffer when you can't find answers. Never give up. We were helped by so many strangers who had gone through the same things…look for the helpers. Today my daughter is healthy and strong and we are so grateful. Cured of the C. diff. and she eventually outgrew the POTS. Get to Mayo if you can't figure it out. It's not an easy week, but you get answers from the best.