Suggest Remedy For Red Blotches And Welts On Legs And Buttocks

My 18 month old daughter has red blotches and welts on various areas on her legs and buttocks. They started last night and then when she woke this morning there were more. The ones from last night seemed to be fading leaving red lines extending the area where they were. Today they are not fading. What could this be? She has her 18 month check up tomorrow, should I wait and ask her dr then our should I try to get her to see a dr today?