Could it be thrush or down to poor hygiene?
Im not convinced she wipes after using the loo when she is at playschool so this could be the cause. I have told her over and over again that she needs to wipe and ask someone to help if she cant. I think she worries she is missing out on something so tries to get in and out of the bathroom as fast as possible! I have put some of dd2's nappy cream on tonight as she looks quite sore.
I could take her to the Boots walk in clinic tomorrow if it is no better.