I don't know what your other dermatologists suggested, bt the only treatment I know of for your kind of problem is to pretreat with an antihistamine. In other words, 30-60 minutes before shower or exercise, take an antihistamine--Benadryl 25 mg or chlortrimeton 4 mg are over-the-counter, but if these don't work or are too sedating, there are prescription antihistamines like Allegra or Claritin. It's a nuisnace to take these pills, but if they work, it may be worth it. At any rate, I think it's worth a try.
my daugher also has been diagnosed with that. They suggest amedication called "periactin". we haven't tried it yet.