If you have any pets,sometimes they may rub up or pass through an area outdoors near some allergic reacting plant life, carry the allergens on the fur, we pet them and unknowingly have just made contact.
Certain shrubs, especially juniper aggravates the skin,causes hives, as well as tomatoe plants,not the fruit.
Or like some have mentioned,soaps,laundry detergents,etc. can also be culprits.
It may be a good idea to see the doc if it persists.
Many years ago, I had this weird itchy rash, mostly on the trunk and arms, turned out to be some sort of airborne virus,that passes, but having shots and special salves speed up the part of itching, turning 3-4 weeks of this annoyance into only 2 weeks.
I can't spell it, but it was called pittiariosis rosea, it really was nasty.
I couldn't believe there were airborne viruses, like common colds, that can do this to the skin.
More I think of it, see a doc or better yet an allergist.
Hope all works out, take care now.
DE