I have had hens doing those mustard poops so I looked it up at the time.This gives me hope!
Some more information.
It's just this one bird -- as far as I know. I look at night to see who roosts where, but they're down and waiting to come out when I go to open up the coop, so I don't know if they've moved around.
They all eat the same thing (Kalmbach 20% flock maker), oyster shell and crushed egg shell on the side. The main "treat" is their feed, wetted into a mash; BOSS about 1-2x a week, and rarely anything else.
I haven't seen any "Gee, I am not well/don't feel good" signs, nor any other indications that she isn't up to snuff.
I think she's laying; I have seen her in a nest box since they started back up. With the cold temps, I go out more often to gather eggs. She is not one to lounge in a nest box, so not seeing her in there regularly doesn't mean much.
If anyone has any more ideas, I'd love to hear them.
Other than what has already been mentioned , but less likely for chickens who don't go outside, avian spirochaetosis (= contamination by a bacteria called brachyspira) and colibacillosis ( contamination by e.coli). The last could be the cause of reproductive disorder.
And of course coccidiosis which also does not seem very likely in your hen's case !
This is a repost for thirsty Thursday but these two terrors are so cute.