Sunburn-like rash

[lollipoppi]

My DS(4) has a rash all around his legs and bottom, which I can only describe as looking like sunburn. It starts at his waist and goes down to the middle of his thighs and it's hot to touch.

He is fine in himself, no temperature and it's not itching him.

No change in washing powder, he had trousers on all yesterday and hasn't done anything unusual like rolling in grass that could cause this?

Is it spreading?

Dd1 I thought had bad sunburn over her shoulder and arms. Then it went down her thighs. Wasn't ill in herself at all. Phoned the GP and they said it sounded like an allergy.
Then she had it all over... followed by dd2 who was much younger and ran a huge temperature so I took her to the GP and it turned out to be slapped face. And he then retrospectively diagnosed dd1 with it too.