Why Do I Get Small Red Bumps On My Skin In Warm Weather?

every year when the weather gets a little warmer my skin starts to get itchy and small red bumps appear..usually on the back of my hands and forearms, legs and face sometimes neck and chest but appear anywhere randomly. they are usually clustered together. i haven't applied anything different to my skin that i would normally use, and i have no known allergies. this only happens in the spring/summer time, and it doesnt even have to be really hot. it happens even when im not in the sun. what could this possibly be