Her bottom does look pretty reddened. Do you think one of the others has been pecking her? That's what it looks like to me. Look around her vent and into the base of her feathers and try to look for lice eggs (yellow bits) or specks of dirt which may indicate mites. If her butt is poopy, you can take some scissors and trim away the poop on the feathers. There is a condition called vent gleet (not saying she has this) that will cause white and yellow patches on reddened skin around the vent. This is a yeast infection.
I've been dealing with some yeast infections and a vent prolapse due to laying issues so your thread caught my eye. This doesn't look like what I've been dealing with ... maybe that's reassuring?
I recently read an article written by a vet that "vent gleet" is not always a yeast infection, it can also be due to a bacterial infection or parasites. And I believe the same can be true for a prolapse, it can be from an egg, yeast, bacteria or parasite.
-Kathy