My 21 month old has a red, raised, blotchy rash on his face, arms, and legs. It began yesterday with one small red bump on his cheek and progressed today to a rash that also covers most of his arms, legs, and face. I gave him a small dose of Benadryl before bed past night and the rash was gone by morning. It then reappeared by afternoon but also disappeared at nap time, but returned again in the afternoon. Any ideas?