The other thing it could be is food intolerance with a low threshold of tolerance. I am thinking that she could have an issue with the whole salicylate group - all berries, stone fruits, citrus, grapes, coconut products, certain veges like tomatoes, broccoli, olive oils, peanuts, avocado, corn. If so, after eating a certain quanitity of any of these foods she reacts. All contain salicylates, and it is the build up of the chemical that matters - so you could eat only a small amount of strawberries, stone fruit and avocado but added together its a huge dose of salicylates then whammo! A reaction! Salicylates do burn the bum...
The intolerance reactions are often similar reactions like allergies although not immediate everytime you eat them. If she is sensitive, then she won't need to eat much before her threshold is breached. Anyway, something to keep an eye out for... an allergic reaction is also very likely - as PP have all said.