This happened to my daughter 2 years ago, when she was 15 months old. Not really hives per se (they were not raised) but just red splotches all over her body. And she was NOT itchy. I took her to the pediatrician who asked if she had had a cold recently. Why yes, yes she had - a couple of weeks before. Everything else was the same - no new foods or clothes, same laundry detergent, etc. The doctor said it was probably viral and related to the cold and it would get better on its own. And it did in 2 days, no Benadryl needed.