My 20 month old son has a rash and no fever, do you know what it could be?

I took him to the doctors on Monday because he had blackish stools. They came out either in hard pellets or in a black pastey form. I only made appointment because my husbands insurance hotline told us too. They did some blood work (still waiting on results) and took a stool sample (also still waiting on results). He never had a high fever, although it did reach 101.4*F over the weekend. Last evening when I went to give him a bath I noticed a rash that I thought was prickly heat. He woke up in the night and it had spread across his chest and legs and looked more like welts or blisters and they itched. It was worse by morning and I took him to the doctors. I umed it could be chicken pox, but the doc said no. He only checked his temp and in his mouth. He never examined the rash other than me showing it to him when he walked in the room. He said to wait until the test results before I give him any medication. Any idea's?