If she didn't have a fever before hand, I'd say it wasn't roseola. Have you looked into food allergies? We're pretty sure my 6 month old son has an allergy to the proteins in milk - he has had a rash for over two months now (and was just fine before that - it came on very suddenly) with everything being changed - detergent, soaps, lotions. I finally cut milk out of my diet (I'm breastfeeding), and a few weeks later he was doing great. Then, I ate some frozen yogurt (at least, they said it was frozen yogurt at the ice cream shop) and the next day he looked horrible again. I've refrained from milk again and his skin is already starting to clear up.
If the rash doesn't go away, you might want to look into some allergy testing, not only for foods, but for other chemicals in detergents and lotions too. Good luck.