I get this when I'm exposed to cold weather with short shorts on in the winter,

"Allergic urticaria on leg in the form of hives induced by cold. Cold urticaria (essentially meaning "cold hives") is a disorder where hives (urticaria) or large red welts form on the skin after exposure to a cold stimulus. The welts are usually itchy and often the hands and feet will become itchy and swollen as well."