6 year old son has spots on his bum cheeks

[Luciemariee93]

hi I'm looking for some advice, my son has these on his bum cheeks I thought they was from wearing pull ups of a night but he hasn't wore them since May. any ideas please?

Is it a sweat rash ?
Does he have a waterproof sheet on his bed or are his PJ’s not cotton ?
Have you changed your washing powder or anything like that ?

yes I've got a waterproof sheet on & no change in washing. my moms said sweat rash too thank you