This could be a solar urticaria or another condition known as polymorphous light eruption (solar urticaria is characterized with an acute onset of symptoms shortly after sun exposure, minutes, and PLE is characterized with delayed symptoms after sun exposure).
Lupus erythematosus must be taken in consideration before a sun urticaria is established as diagnosis.

Best wishes!

It's probably the sun, but do you notice this when you go outside normally? Like when you spend a day working out in the garden, walking around between stores shopping, or something? Does it happen then?

Do you do anything else unusual when you go on holiday? Maybe you are allergic to chlorine in the pool, or sunscreen, or seafood. But since you may not be exposed to those things at other times, especially for several days in a row, you don't notice. Just a thought.