Have she been tested for allergies? When my DD turned 2 we got her allergy tested. First the blood draw test and then the skin back scratch test. I suggest getting both done since they detect different allergy reactions/potential.
My Dd also has eczema, with an allergy reaction. She would also get hives, both itchy and non itchy, after eating certain foods.
I also recommend the food diary. After getting my DD tested, we did the food diary. And I keep a mental food diary for myself and my allergies, since NONE of my food allergies show up on the blood test. Both peds said that the tests are not 100% fool proof, so the best "test" is the food diary and the elimination diet where you temporarily eliminate potential allergens for 2 weeks and then add them back into the diet one at a time (about 1 allergen every 3-4 days).
Also check every label of products that come into your house, both food and non food items. Manufacturers often change ingredients. So you could be feeding your daughter something that she has had many times, but maybe there is one or two ingredients that have recently changed. Also, like another poster mentioned, you can become allergic to something that you have already had.
My DD can only use chlorine free diapers and wipes because the chemicals in other diapers give her a rash. So maybe that's something you want to look into.
I have always had food allergies. But over time some have changed. As a child I was allergic to chocolate, almonds, apricots, carrots, and oranges. I am now allergic to all of the above, plus peanuts, wheat (I am technically not allergic, but it messes up my digestive system), shellfish, and also lemons. But I can now tolerate cooked carrots.
My DD is allergic to wheat, peanuts, walnuts, cashews, oranges, fresh pineapple, and strawberries. We have not given her shellfish yet. Her blood test showed allergy to wheat, soy, eggs, walnuts, and peanuts. Her backscratch test showed only a reaction to cashews. Somehow her body has come up with a way to tolerate eggs and soy so she can still eat these.
I'd say that if your daughters hives don't bother her, leave it alone. I only put the hydrocortizone cream on my DD when they itch her enough that she complains about them as it's actually bad for the body. Hives are just another way that the body tries to get rid of toxins/allergens. My daughters hives look like bright red, purple, and blue, raised bruises. They look HORRIBLE when she has them. Sometimes they itch, sometimes not. Then sometimes she gets the little red, itchy bumps.