I was polka-dotted for a while back in January (wow, my Horribles queue is getting a bit ridiculous). It was caused by some type of bug bite. It could have been fleas from the iggies or something more mundane. Luckily the hives didn't spread to my face. It was a bit weird looking but it didn't itch until it started to go away. Then it was a few days of torture.
I'm happy to report that I'm polka-dot free now.