I think those doctors were right. Rashes in kids are quite common. There is exczema which is very common, and then som more acute skin problems.

The one which sounds like what you describe, I think, is, but a bit scary and unpleasant. Most of the skin on your body turns pinkish or bubbly. In Swedish, it is called "nettles-fever" after a stinging nettles, you know. It's as if you fell into a bed of nettles.

Apparently, the latin name is Urticaria.
Urticaria - Wikipedia, the free encyclopedia

I got it once, for no obvious reason when I was at another childs party. Then another time after swimming in a lake that was overgrown with algae and then a mild case as an adult, after taking penicillin (so now I am classified as allergic to penicillin...).

It only lasts a couple of days or so, and all you need to do to cure it is drink a lot of water and only eat something that is guaranteed "allergy safe" like rice or potatoes.