Wife had this problem about a month ago. It may sound weird, but take a hot shower, get all the crap you have put on your back off, then lay on a heating pad. You have what is known as the "Hell itch". You need to use heat to help the itching.
Here is a nice link to people discussing the problem.
http://www.medkb.com/Uwe/Forum.aspx/...n-extreme-itch