The only rash that comes and goes in minutes is hives, or urticaria. These can be large, red welts, but they can indeed look like what you're describing. Only a very small percentage of hives are allergic. Most have no cause, come and go for a few weeks, and then burn themselves out, just as the dermatologist said. I generally tell patients not to tie themselves in knots looking for foods, detergents, or other factors, nor are blood tests likely to be productive. Just treat the symptoms--Zyrtec and topical anti-itch creams like Sarna or Benadryl are fine. And please don't be nervous or scared about this, because you don't need to be.
Best.
Dr. Rockoff
Hi!
I have been experiencing a similar type of situation over the past week. My legs have been breaking out in what looks like a SERIOUS rash, mostly in the morning and late at night, and itches like crazy! Then, after about an hour, the itching stops and the bumps go down. My mother thinks that it's just hives, but, I have a doctor's appointment in a few days just in case. I've been taking Benadryl for it and that helps a lot.
Thanks,
Josie