What Causes Itching Whenever There Is A Rise In The Body Temperature?

I have a bad body itch whenever my body temperature rises or when i excercise. No bumps are on my body when this happens but my back and head typically get unbearably itchy. This condition has affected me by forcing me to quit my previous job because of the itch. If anyone can help please.... I read some threads on google saying "Chronic Uticaria" or something like that but it says that bumps are visble and I dont have bumps.