Is it thrush that hasn't been treated for long enough and therefore keeps coming back?

Bubble baths? They can be particularly irritating for female genitals.

The laundry powder you use? My DS gets eczema on his buttocks from some laundry powders but not others - it only happened once he was in undies.

You might also want to make sure you use an anti-fungal rinse in your wash (Canesten or Dettol) while treating her rash.