Glad to hear she's doing better. The neck jerk sounds like she's adjusting her crop , normal maneuver. You can try mixing the crushed tums with a small amount of scrambled egg, if you really can't get her mouth open. Sometimes it just takes a bit of practice to get pilling down. Also, you can trim her butt fluff with scissors to help keep her clean. I agree with Eggcessive, try to get her out where the flock can see her, and she can see them . If there is room in the coop, just carry the crate and hen out together, or put her and the crate in the run, if it is sheltered. Then you can bring her in at night if the run is not secure. Do what works for you, keep yourself safe if the rooster is a concern.
I've left her with her bestie, telling the SG to take care of little Sweet Pea and not to leave her. I didn't think she'd listen to me, but others have in the past, so I gave it a shot. I stayed with the flock until I saw how Sumiré (the SG hen) would act. At first, she was in the process of laying an egg when I brought out the crate. She came out long enough to see her friend and then laid her egg. Now Sweet Pea is laying in the nesting box full of eggs and Sumiré has taken up residence in another box. She's sticking right close to Sweet Pea, and baby girl seems more content. She doesn't look like she's in pain, just not feeling too great. I'm going to try the tums in a peppered scrambled egg for dinner. I heard on here that pepper is very good for chickens and it'll hopefully mask the smell of tums. I have the berry flavored ones, and I think I messed up by not buying the original.

I was thinking maybe it was her crop. I know that I've seen others do it, but where she was sick I didn't know if it was going to be gapeworm that just wasn't at its worst yet (I'm always prepared to tackle the worst problem imaginable so the easy problems don't take my sanity).

Her belly isn't gurgling anymore, so that's good, I think. Can chickens get indigestion? We get indigestion if we eat bad for us foods, so maybe? Google seems to be all over the place with an answer.

My rooster is only ever a threat if I make her scream, then her big King tries to rescue her from mean old mama. I think they remember that the last chicken that I caught never came back.

But so far this seems to be the most perplexing case of problems that I've encountered, including those I've had with small children! This one just had me flummoxed every step of the way.
Ok. I have seen no respiratory issues with anyone in the flock, so that's a definite no. None of them show signs of parasites, internal or external. Everyone, including Sweet Pea, have clear, bright eyes, no discoloration on their combs or wattles, no swelling where there shouldn't be (some of the little gluttons have full crops right now), no missing feathers on anyone, no pecking or excessive bullying (only occasional pecking order usualness). There's no sign of a fever or sneezing or gasping or anything. No odd droppings or smells. No scratches, peck marks, or any other wound, healing or fresh, on any of them. They all seem extremely healthy with this one exception, and right now she's just sluggish and sleepy and that's understandable for not feeling great; but all in all even she has few symptoms as of right this minute.

Thank you!

