Hi F.,
I've had hives twice - once was a severe case that turned out to be an allergic reaction to an antibiotic. The other was a mild but persistent case that disappeared on its own after a couple weeks, and I never figured out what caused them.
In the mild case, yes, I had ones fade and new ones appear. It is very important to watch her closely, in case the allergy gets serious. In my case with the antibiotic, the hives got so bad overnight that hives were actually inside my windpipe and caused me to wheeze (closing off the airway slowly) and lose ability to stand up (because I'd faint), so I was taken to the hospital. I would just check on her once or twice a night if possible for severe puffiness and smooth breathing, just in case.
As I understand it, hives is always an allergic reaction to something. The hard part is finding out what that "something" is... it could be a suddenly developed allergy to the cat, or to her bath soap, or anything. I've read it's best to eliminate all possible substances and then re-introduce them one at a time. Or go for an allergy test, though they can be quite expensive (and I've heard they are sometimes not too trustworthy). I hope this helps!