When children of her age ( up until the age of 12 I think actually ) have any kind of throat infection, ear infection, chest infect etc... you get the picture... they also often complain of tummy ache. INfact, it can be the presenting ailment. They have a gland that comes up, low down in the abdomen, which when enlarged, gives them 'tummy ache' Its just a galnd which would come up to fight infection, much the same as any other, so nothing to worry about in itself.
Its difficult to sat what it could be with such little info ( I know you've said what you can! ) and not seeing her. oh, and not actually being a GP! but, I do have paediatric experience as a nurse - so am talking sense I promise!
So long as she is well hydrated, not eating is a symptom of what ever is going on for her. You've checked for rashes etc, and she's not complaining of a stick neck or sore neck at all with the headache? Can she touch her chin to her chest?
If no to the latter question, take her straight to GP, and tell them her sypmtoms on phone especially htis one .
Stiff neck with headache, and not being able to extend the neck down to the chest is a sign for meningitis, of course i'm not saying this is what the problem is, just that if this is present then it possible.
Its equally possible, and more probable that it is a virus. Horrible nasty little virus's going around currently, this time of year breeds some nasty ones. Bless her. Keep the fluids going and take to GP.