Hi, foxy - I've got "cold urticaria" - a skin reaction to moist cold. I developed it 5 years ago - on a hot summer day I was holding an ice cream sandwich when my hand started to itch and swell, and it developed red hives and welts - totally out of the blue! I called my pharmacist, and she advised me that as long as I wasn't having trouble breathing/swallowing, to take fast-acting Benadryl, which did indeed help calm things. A few hours later, the symptoms had disappeared.
When I went to the doctor (I still didn't know what this was), I bought a McDonald's shake and held the cold cup against my forearm in the doctor's waiting room. In 5 minutes, the arm was red, itchy, etc. The doc said this was a crude but effective way of nailing down the allergy. He advised me to steer clear of icy drinks, cold showers, etc. and gave me a script for an epi-pen in case I had a more severe reaction. (But I never did.)
In the weeks that followed, I met a woman who had hot urticaria - she broke out in hives whenever she took a hot shower or washed dishes without wearing gloves.
So it may not be something you've eaten to cause this reaction. Search for urticaria on your favorite med website (I'm in the U.S. and use www.webmd.com) and you'll see that this can come about from exercise, sweating, sun exposure, etc.
Whatever the cause, I'm surprised the antihistamines aren't working for you. Maybe see about getting something stronger, and keeping some kind of log as to when the rash appears/what you were doing before it came.
The bad news is, it may take a while to narrow this down. (Unless you visit an allergist, who could do skin tests.) The good news is, you may grow out of it. Mine developed in 2001 and in the past 2 years the reactions have been getting rarer and less frequent. (Of course, I haven't jumped in any ice ponds, either.)
Good luck!