I don't have much experience, but my first thought was gleet maybe? Actually, my first thought was "Eeeewwwwww!" so gleet was a close second. From an old BYC post: "Vent gleet is a yeast infection that causes some nasty, stinky, white discharge."
Try feeding her some plain yogurt and soaking her bum in warm water with Epsom salts added - can't hurt, might help.
Check out some pics of vent gleet to be sure though. My first thought was a prolapse. Has she been laying recently? Seen any bright red blood? Cuz that looks like an injury with dry blood and possibly trying to heal itself.