Red patches all over babies body

My baby is 20 weeks old and over the course of an hour I have noticed these red patches all over his head including behind his ears and his face, his chin and neck, the back of his neck, his full back, his bottom, and parts of his stomach. His legs and arms are completely clear. His salmon patch on the back of his neck looks a darker shade than what it was. He has a temperature of 37.1 degrees (higher than usual but no fever) and his behaviour is fine.

I have called the doctor but I have been waiting for a call back for ages. Anyone have an idea of what this could be? He has had heat rash previously and it’s been red and raised, nothing like what this looks like.

He hasn’t started solids and he’s exclusively breastfed. Thank you.