I'd check the really obvious things first. Have you changed your washing powder, that the bedding has been washed in? Fabric softener? Food?
I know strawberries make one of my kids come out in spots like this, I'm wondering if she's had anything out of the ordinary to eat, or even if you or your partner have been wearing perfume/aftershave that's caused an inflammation?
Other than that I'm useless, I hope you find out what it is.
Shona