Sorry, time4, I didn't realise that. I do really sympathise - one of my DC was ill for 2 years before we got a coeliac diagnosis.
Have you heard of 'Non-Coeliac Gluten Sensitivity'? There is some info on the Coeliac UK website about research into it - it has similar symptoms to coeliac disease but no antibody response (therefore negative blood test/biopsy). Not all doctors believe in it, but recent research suggests that it is a real condition, and Coeliac UK is sponsoring a further study, I believe.
One of my DC is full-blown coeliac. Another had all the same symptoms but negative test. The paed gastro said it could be that he was just at the initial stages of developing coeliac but too early for it to show up on the test. But it could also be this non-coeliac thing.
I have chosen to go completely gluten-free with this DC as well as the coeliac one. It's easy to do so when you're already doing it for one person, so not such a big deal for us as it might be for you. All the symptoms disappeared within a couple of months (and improved within a week or so). So I would recommend trying it for your DD.
Coeliac Uk has a lot of info about the gf diet. The main thing is to learn to read labels - quite straightforward now with new legislation. Basically, as long as an ingredients label does NOT mention wheat, barley, rye, gluten or oats, then the food is okay. We avoid all 'may contains' too, to be safe.
Oats don't contain gluten, but commercial oats are all contaminated with gluten from growing in fields where wheat is grown in other years (so some wheat plants will come through) and from processing on the same machinery. You can get gluten-free oats which are grown and processed separately. But coeliacs are told to avoid these for the first year after diagnosis, since oats contain a protein (avenin) which is very similar to gluten and can cause similar symptoms in a minority. So it's best to avoid those as well to begin with (which means you have to check labels even on products marked 'gluten-free' as a lot of gf biscuits etc. will have cheap gf oat flour in. If your DD's symptoms resolve on the diet, you can try reintroducing gf oats and seeing if there's any reaction.
good luck, and sorry for sounding snippy before.