DD6 had a similar rash/hives a few weeks ago. It started out on her legs, then her stomach and back. Gave her Benadryl that night and it got better. It showed up the next day again and gave her Benadryl again. Same thing the next day. I ended up taking her to the DR thinking it was an allergic reaction to something, but the Dr said it was viral. She gave me Zyrtec to give to DD and said it could last 2 -3 weeks. After about 10 days, it was gone.