We've had a similar thing here. Google Roseola Infantum, the Better Health Channel link that comes up first is what the Dr gave us yesterday... It's basically a rash that comes out at the tail end of a virus, as the temp starts to return to normal.

Unfortunately for us, we think J's rash is actually Rubella because she didn't have the temps before & the rash is still there & not fading after 24 hours like the Roseola one does.