When I heavily use muscles that haven't really been exercised in
a few weeks or more, I get very sharp pain in
those muscles the next day or two. That's when I know the hives are going to come. Unlike most exercise-induced hives I've read about, the hives don't come when I exercise, but a day or two later - always after the sharp muscle pain. The first few times I got them they got so bad I ended up in the emergency room where they gave me epinephrin, etc. and they went away.