I had a nasty case of hives that lasted a week about 10 years ago. Doc thinks it was stress induced. I was under a great deal of stress having to deal with the builder building our new house who was very difficult to deal with, not to mention having two small children at home and a deadline to get out of our current house knowing the new one wouldn't be done in time. They were so bad they would wake me up at night and only relief was to go stand in a cold shower. I've never had them since.

Here is a site with some other possible causes http://www.quickcare.org/skin/causes-of-hives.html