DD has IBS too (but specialist hasn't yet ruled out Crohns) - her stomach is BADLY affected by the following & according to her hospital, these are common triggers -
Chemical food additives - azo dyes, benzoate preservatives, MSG, & artificial sweeteners etc.
Lactose in milk/cheese etc - when well, she is fine with dairy so long as its lactofree, or mature or cooked cheeses etc -
& Stress is also a bad one, that can be harder to control :( ( is your DD worrying about SATs or anything else??)
During a flare up, diary in general becomes hard to digest & makes her worse, as does any wheat, & meat can make things worse too - (though DD never eats meat)
Also during a flare up keeping her on bland roughage, wheat & dairy free foods helps her tummy heal much quicker - as does adding the spice Tumeric to what ever you do give her, its a natural herbal medicine for inflammation of the gut - I cook rice in it & she eats the rice :)
I also make sure she gets plenty of extra "tissue salts" if she has diareah she will lose these important electrolytes - you can get powders, but its hard to get DCs to take them, so I give my DD crisps & coke - of all things, they are good for replacing the important pottasium, sodium & glucose - if these are low as a result of diareah - it can make them more stressy & cause more stomach problems.
Some other food groups can cause problems, such as the nightshade family, that includes peppers etc, so it might be worth keeping a food diary if the above doesn't work.
We also have ended up in A&E with suspected appendicitis - turned out to be Colitis - AKA an inflamed colon - which can have several causes - including IBS, Crohns etc
Good luck finding her triggers, it can make a massive difference to getting it under control :)