Please help me identify my baby's rash?

I cannot include a pic, but I will describe as best as I can. He is 5 months old, he had a fever Friday, took him to doc, said it was viral. Fever was gone by late Sat. Now, he woke up this morning, and what I thought looked like spider bites or mosquito bites on him. (He has nto been outside at all). But, the "rash" will disappear and pop up somewhere else on him. The only meds he is on is Nystatin for his thrush, which has never effected him before. The rash looks kind of like a slap mark with a little puffy mark in the middle, like a mosquito bite, and is warm to the touch. Like I said, I ruled out bugs, b/c it seems to just "move around" on his body. Right now, it is on the front of his legs. I have not changed detergent. What do you think??