Just on the food thing, which is possible (as in it's possible with any person, I am not diagnosing your child over the internet!). If she does have a food issue, depending on what it is, and there are many possibilities, chances are it will be too hard to pick up by eliminating a few obvious foods or drinks.
Coffee for example - if it's cafine that's the issue I don't know how long it would take to see an improvement, ColinsMum could be correct that it would take only a few days. But Coffee is also a strong Salicylate, if she has a Salicylate issue it will likely take longer than a few days to see a change AND you would have to eliminate all Salicylate, and coffee is just the tip of the iceberg there.
We missed my youngest DDs food issues for a long time because I tried just eliminating dairy and it didn't help much. Which would be cause she needed to be off Dairy, gluten, eggs, soy, salicylate, aimines, colours, preservatives, flavours. Once we got her diet right she stopped screaming, started sleeping, all manner of things got a lot better. And we KNOW when we make a diet mistake.
