I Don't Know If You'll Have An Answer, But I

I don't know if you'll have an answer, but I think I may have some form of reaction/condition. This started fairly recently, within the past 3 months, but it seems to start whenever I begin to sweat, only a little bit though. I get a really bad pricking/itching sensation mostly on my upper back and upper arms. It feels like I'm being pricked by a bunch of needles. By begin to sweat I mean possibly walking briskly for a short while, or if I get startled, or something shameful embarassing happens. Any idea?