I am not a medical professional, but it sounds like exercise urticaria. That's a type of exercise induced allergic reaction. I tget it sometimes, too, usually when I haven't worked out for a while or when I do a very tough workout. You can take something like Benedryl or even use the cream. A cool bath or shower helps me. Talk to your doctor about it. In most people, it's non-life threatening. You can find ways to cope with it, like the methods I mentioned before or doing a gradual and gentle warmup.