For me it is heat rash in the same places as yours but also in other places and the only thing that cures it for me is a weeks course of Prednisolone. I have tried every tablet and cream prescribed but nothing works. I have seen a dermatologist who says they have millions of people who suffer from a rash and they never establish what's causing it. I was told it could ant bites, the die in my tights, cats, you mention it, they have no idea. I have an emergency kit at home, so I can start taking them the minutes is appears. Once you start scratching it just spreads like mad. I wear cotton gloves in bed at night alway as I scratch like mad. It can happen in summer or winter, summer from hot weather winter from central heating and over heating in bed.