My 4 week old baby has a rash that is spreading. Any ideas on what it is?

I know it's not just baby acne. He does have a little of that on his face. But his whole side and back is all red and slightly bumpy. It's spreading to his arms and legs...pretty much all over. I have an appt. with his doctor tomorrow, but wanted to see if anyone had any ideas. I have 2 cats and wasn't sure if he could be allergic to them, or to laundry detergent, etc.