Rash on baby

[Cakeandmarshmallows]

Hi there
I know that this is not a medical forum but I'm calling on the good people here for advice. My 5 month old son developed a rash in his tummy and chest on Wednesday evening. Very small pin prick marks feeling slightly rough to touch. Took him to gp on Thursday who was not sure but suggested scarlet fever!!! But it's not gone red at all, and he's well in himself, happy chatty and no temperature, gp checked and I have been checking. She suggested plenty of moistureiser and to keep eye on it. Since then It does seem to have spread onto his back and sides and his neck a bit . He's still well and happy so I'm at a loss about what it is!! Any one for any ideas?
Many thanks.

Have you changed laundry detergent lately? If it's a reaction to detergent it can take several days to go. Maybe try some e45 anti itch cream?

Also there's something called prickly heat rash which they can have and is fairly harmless.

Thank you both. No laundry or other changes like that!! But yes I did wonder about prickly heat. Thank you both.