The Mysterious Vanishing Rash

I know I've been blogorific today, but here's a new (and quickly old) developemnt: I posted earlier commenting that it looked like my son had a bite on his face when he awoke from his nap. He went back to sleep (finally) and when I got him up at 4, the original welt was gone, but there were many more on his right cheek (face, not butt). Then, I realized that the welts/rash were also all over his right thigh and hip, all the way down to his knee. I called the Doc thinking maybe a reaction to the shots from yesterday. The nurse has never heard of such a thing, says she'll have Doc call me back. When the nurse calls back at 5:40pm, all that was left was one small spot, which had been covered by his diaper. I'd put Neosporin on his cheek (i thought it helped with the first spot) but just left his pants off so that his legs got some air.

Have any of you ever seen such a thing?? I'd like to AT LEAST know what it was!