Hives, Swelling, Prior fever

My Daughter is 7. Yesterday she broke out in hives and her lips swelled up huge. I ran her into the dr. and he perscibed Zyrtec and Prednisone. It helped. Today a few hrs before I had gave her medication yesterday she broke out in hive from head to toe like the day befor and her lips started to swell again just slightly. I gave her the medication again and about a 1/2 hr later she was a lot better. After a couple of hrs. they were back again with no swelling. They since have calmed down again. Nothing has changed in her diet, or any of our soaps and she has never been allergic to anything. A few days before this she had a fever that went a way with no other symptoms. I am worried about her. Should I be. When should I go back in to the dr.