Heat rash from nappy?

[Igglybuff]

We switched DS's disposable nappies as he seemed to react (bad eczema on his bum). This morning we noticed he's got a bright red mark on his inner thigh - warm to the touch. It's only on one leg. Could it be the different (disposable) nappies? He's been wearing them for a few days so seems strange to react now. Just wondered if anyone had any ideas?

DS1 and DS2 both had heat rash from the reusable ones. The HV said that they were struggling to regulate their body temp with such leak proof nappies. I tried again at 6 months and they both still had heat rash. I ended up giving them away and sticking with disposables by the time i had DS3!
None of them ever had problems with disposables though, but in fairness i kept with the same brand.
I should also add that they are naturally sweaty little souls - always stripping off to t-shirts whenever possible!