4 month old rash?

[Starry4321]

Hi everyone looking for some advice. My 4 month old baby has a rash on front and pack. It’s like there’s some dry patches and tiny pimples. I’ve noticed it gets redder when he’s been in the bath or warm. It does come and go though. Some days it’s almost totally gone. Other days it looks a bit worse. Took him to gp and she wasn’t concerned, she just said it would go away. He’s EBF. Anyone know what it might be?

I would keep a really detailed dairy given it flares up and down to see if you can see any patterns. Include any creams, soaps, changes in washing powder as well as a food diary for you in case of allergies.
My daughter had a food allergy which caused a rash to flare up where as my friends son was allergic to oats and reacted to the baby alveeno she was using on his dry skin.