This summer, my son suddenly became allergic to Red Dye #40. Rather, his allergy suddenly changed from an external one (it made his nose itch if he got it on his face) to an internal one -- even small amounts made him itch on one portion or another of his body, and even all over his body. It took him about half an hour from ingestion to itching, and about a day to clear it up. Do you know how many foods have Red 40 in them??? Most types of candy, and many sports drinks, cereal, and even ice cream have it in them -- basically, anything red, orange, purple or pink will have it in there, as well as many yellow and brown things -- even some brands of *white* frosting have it, to give it a slight beige look. And it's not limited to foods: the first thing that he had that made him react this way was a motion sickness pill that had flecks of red in it, most forms of Benadryl (brand, generic, tablets, capsules, or liquid) has red 40 in it, and even chewable vitamin C often has it in there! The things I reach for first, for an allergic reaction (vita C for mild reactions, Benadryl for worse ones) would make my son's reaction worse!
Why do I bring up my son's allergy? Just as an example of a common ingredient that is in many things, including things that you wouldn't necessarily think would have it, that could cause a reaction like this. And because it suddenly changed -- one week he was able to eat stuff with Red 40 in it with no problem, and then the next, even a tiny amount made him itch.
So, I would suggest keeping a food diary of when she eats what she eats, and also keep track of the detergents and other body products that your daughter may come in contact with, and also keep track of what time of day the hives break out, and see if you can find a common denominator. It may be that the Lucky Charms she has been eating all her life is now making her have hives, because of the Red 40, Yellow #5, or some other ingredient in it. It's possible that the strep triggered an immune change, or lowered her body's system/defenses so that she is now vulnerable to some ingredient (in food or detergent or soap) that she was fine with before. [We moved halfway across the country right before my son's Red 40 allergy changed, and I think that may have triggered it somehow -- something about this climate or something may have changed him.] But if you keep such a list/diary, you may be able to track down the cause(s).