DD, 10, has a rash. What could cause it?

[BethAndHerBrood]

DD has a red, blotchy rash. It's mainly on her thighs,goes from just above her knees to where a pair of shorts would end, IYSWIM, slightly on her feet. Quite itchy, not hideously so. No other symptoms.

What could it be?

I'm hoping it's one of these things that looks worse than it is.

Has she been outside? It's probably either prickly heat or she has brushed against a plant which she has reacted against.

Will probably be gone by the morning.