Rash - picture - any ideas?

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Dd (4) had a red mark on her bum a few weeks ago then it went all shiny almost like a burn. Now it's turned like in this photo but also she has same rash on other 'cheek'.

Any ideas?

I'd say that looks a bit like an allergy rash from something on the skin.

Thanks - I had wondered about that but it's been going on a good while now and also she's on daily antihistamines for allergies already...

Is it itchy? Are the bumps like little fluid filled blisters? It looks a bit like the rash my child had before being diagnosed with coeliac disease. It's called dermatitis herpetiformis. The coeliac UK website has more info but says it is often found on elbows, knees or buttocks and can be symmetrical on the body.

If it's itchy possibly an allergy. Seems a bit extreme for a heat rash. Looks like a skin infection.

Isn't itchy.
It is still there but has settled down. Not as raised and not as red.