For many, food allergies will show up in childhood. However, most conventional doctors are not trained in nutrition and so are unaware that food allergies will provoke hives, acne, atophic dermatitis (looks like acne but is a type of eczema), plus a whole host of other chronic illnesses. And if a person is only mildly allergic, or just has an intolerance/
sensitivity to a food or food product (such as dyes, colorings, flavorings, MSG, etc.), it can be anywhere from three days or longer for any symptoms to show up, making it hard to know which food(s) triggered the reaction. And if she's allergic to more than one food at the same time, it's hard to do a food elimination diet. I tried one of those, but because I have too many allergies, the results were inconclusive.
After being very sickly all the while I grew up, to the point of starting to acquire chronic illnesses as an adult, at the age of 40 I finally discovered that I have an intolerance to the proteins in dairy (casein), soy, and gluten/wheat (gliadin). Even after cutting them out of my diet for two weeks (I just found out), I am having a horrible outbreak of atopic dermatitis on my face. I have since gone back and had another food allergy test taken; this time, it's a blood test and will check for 100 allergens (the intolerance test was a saliva test). I have had to see an alternative doctor (I see an Integration/Function doctor, as she is also a MD), and am paying for everything myself, as my insurance company doesn't cover her and won't let me see her as an outside provider.
I am sure the test will come back with more allergies, because I'm also having hives again (however, it's winter and cold and my hives are contact hives, which don't like the extremes of hot and cold). I'm concerned it will say I'm allergic to corn, as I've been eating a lot of corn lately (can't have wheat, rye, barley, or spelt). It should also come back positive for the mint family, as I break out in hives if I eat something with pure spearment, pepperment, wintergreen, creme de menthe, etc.
At the least, I would check her for a food intolerance test (IgA). That one is a saliva test and checks for egg, soy, dairy, and gluten, the main, major allergies.
Having had hives all my life, I can attest to the fact they aren't any fun. Finding out if she has food allergies will help in the long run, because if the foods are avoided, her immune system won't get run down from her body "fighting" against the inflammation the allergen will provoke in her body.
Good luck!