3" welts randomly appear instantly on back only and disappear with in a mintue never change anything use same products all the time they randomly appe?

Fleeting welts: Those welts may be hives which are itchy and an allergic reaction to something internal(like a food or medication) or to something external like temperature,exercise,pressure on skin for example. Hives usually take hours to resolve ,however. Another possibility would be insect bites which last longer than your welts. It would be helpful if a physician could see these welts for diagnosis.