Seashell, we need more info ...
How old is she? What does she eat in a normal day? Is she breastfed or formula fed? Are there allergies in your family? Has she had any other reactions like eczema, diaper rash, vomiting or even cradle cap?
My son has thrown up on caramel colouring, so I looked into it. Sometimes it's just burnt sugar, but other times it does have dairy content, and that's what my son is allergic to. Or it's sometimes cross-contaminated with nuts (another of my son's allergies).
It's also entirely possible to eat something on a regular basis for months or even years, and then become allergic to it overnight. So you'll have to keep a food diary to track EVERYTHING she eats for a few months.