Hives like rash that disappears when warm.

Help! My 15 months old gets terrible hives on arms, hands, and legs. Doctor said it was just a contact dermatitis and could be from anything and she would grow out of it. The only thing consistent about it is when the air is cool (even air conditioned) it pops out. The colder it is, the more spots come. Sometimes when she wakes up if her diaper is full she has the spots where the skin is cooler from the pee. It has been going on for a month now. If I keep her totally covered there are no spots. It looks like scary welts all over her and then I put her in a warm bath and 15 minutes they are pretty much gone. I am baffled. Does this mean she is having an allergic reaction to cold, or could cold be aggravating her allergic reaction to something else? And advice would be great!!!!