Wow, that's awful. Allergies can be pretty rough and prednisone....not a fun drug to have to take. Don't be surprised if Destiny (what a beautiful name!) get's a little irritable while on that stuff. Lots of side effects, but the drug works very well for allergic conditions. Is there any sign of asthma or rhinitis? It's VERY important to watch out for wheezing and to take her to an ER right away if that were to happen.
She needs an allergist right away. If she took her last dose of prednisone this morning and is already getting sick again, she really needs some help. She probably needs to be on an antihisamine like Allegra. The most important thing is determining what she is allergic to and an allergist will do skin testing, which is relatively painless. My daughter just had it done last month. It was very easy. One important fact about skin testing...you have to be off all antihistimines for a few days before you get tested!
Allergies can come on suddenly. You can be perfectly ok with something all your life and then, boom, out of nowhere you're allergic to it. I was midly allergic to cats until I got pregnant...then I developed severe asthma. You just never know. So you must look at everything in your daughter's environment as the potential culprit, even if she never had a problem before.
A few questions...do you have cats, dogs, mice or rats, birds, or any other furry or feathery pet? Is Destiny coming into contact with excessive dust? People with dust allergies can get nasty hives from it! Is she around mold or mildew? Like in a damp basement with carpeting that she may play on? Animals, dust and mold are the most common household allergens. When I was a kid, I got hives from playing on the grass and we discovered that it was the fertilizer I was allergic to. There are so many things that can cause allergies...it's a tough situation to be in. I hope you are able to figure it out. The best of luck to you and Destiny!