ds' cheeks keep going red in patches (when he's hot?) - anyone got any ideas why?

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ds (3.8) gets these red patches on his cheeks that come and go. The flare ups seem to co-incide with him being warmer rather than colder. Sometimes they are really quite vivid and other times not there at all. He's otherwise well but when they are really 'up' it doesn't look right. Anyone got any ideas?

TIA

crikey, it's busy at this time of night!

My DD has had slapped cheek virus, and the rash can come and go for months. She still gets the red cheeks if the sun is out or she is hot.

Ooooh, thanks lou, ds had it a while ago so maybe that's it!