Do you use a glucose meter? High blood glucose makes us more susceptible to thrush or other fungal/yeast infections.
If it's not that, have you tried using something that's used for diaper rash on it? I'm overweight and occasionally get an uncomfortable slight rash in my armpits, in my groin or under my breasts, even without high bg. The stuff you use for mild diaper rash (sorry, don't know the English name for it) works a charm for me, but if it's a yeast infection due to high bg you'll probably need something to cure that.