I'm no expert, by any means, but I have had thrush, and it sounds similar.

You could try going to your pharmacist - they sell an anti-fungal nappy cream that you could use on your DD - I think you apply it every nappy change (so in your case I'd try applying it 3-4 times a day) until it is better, and then I think for a week after?

Has she been on antibiotics or anything recently?

Also, try increasing the natural yoghurts etc that she eats (if you can), or maybe the pharmacist can suggest a suitable probiotic for children?

Good luck.