And to be honest, I have no idea. I'm guessing there's something out there that I'm allergic to. I get rashes on myself if I'm out for long. And I know it's nothing like poison ivy or anything. It's sort of like hives, but on my legs and arms.
The doctor said it was probably the heat, but in your case, I'm guessing it's not because you said it wasn't too hot.
Anyways, I wouldn't worry too much. Just put on some Cortisone. It's an itch/rash cream you can get almost anywhere. And to see whether it's an allergy or not, take a Benadryl to see if the itching goes away. Benadryl does make you a bit drowsy, though. So take it after you start scratching; when you're at home.