Not sure, Katie. You may be describing some sort of sun hives or allergy. The best advice I can give you is to use a sunblock with SPF 30 (not 4), one with UVA plus UVB protection, and ask your docotr ofr a prescription for an antihistamine that won't put you to sleep, like Claritin or Allegra. Take this (or Benadryl if you can't reach your doctor) a couple of days before your vacation, and every day during.
Best.
Dr. Rockoff