You need to sort out her mucky bum before the flies lay eggs on her, you'll need to bathe her rear end to clean it. Is she showing any other signs of illness, gaping, respiratory problems, is her comb pale? Have you wormed them, checked for lice, red mite etc? what's her poo look like? it could be any number of things, they go downhill fast. I would cut out all treats and stick to the pellets with some poultry spice in their water.