Localized itching in one spot that won't go away even with scratching is a nerve thing - or so I've been told when I complain about those. (I usually get them on my feet. Proof that there must be a force of pure evil in the world - nothing that annoying could exist otherwise!)
If your whole upper body is itching, and you don't have allergies, aren't wearing a new shirt, using new soap/detergent, etc, you may be getting dry skin. This crazy weather these past few months has destroyed my skin! Also, once you focus on itching, it can get a lot worse, at least in my experience. I also know that when I'm stressed or worried, I itch like crazy. And, for some reason, when I'm very sleepy, I itch all over.
I'm itchy now just writing about itching! Once it's on your mind, it's hard to shake.