rash after swimming and also sometimes after being too hot

[racheyh]

Hi, My son aged 3 sometimes gets a raised red rash on his arms, tummy and back after swimming and sometimes after running around like a maniac and getting a bit sweaty. It doesn't bother him but it is really hot to the touch (hotter than his skin where the rash isn't iyswim) and in some places especially on his arms it is not so much a rash as quite a big 3 or 4 inches patch of raised red skin not dotty like a rash. Anyone have any ideas what this is and if I should do something or just leave it as it doesn't bother him and fades gradually over a few days.

[scurryfunge]

Reaction to chlorine?

[racheyh]

I was thinking about the reaction to chlorine but then it started happening when he was hot not just after swimming. He sweats loads and his pillow is often wet in the mornings so maybe due to heat is the only way of avoiding it keeping him cool? Someone suggested piriton is it worth it? I don't like giving him medicine just for the sake of it.