It could be thrush or a combo of nappy rash and thrush. You could try some canesten cream, I recommend putting a nappy rash cream over the top to keep it there and also help with the rash. There is a cream Daktozin (I think?) that is both a fungal and nappy rash cream in one, which you could try. With thrush you need to keep on treating for awhile once the rash has gone to ensure it has been treated properly.