Poor baby, she looks like she has a painful or at least full and uncomfortable abdomen. Best analogy I can think of: when your pants are a size too small (like after a holiday dinner!) it's more comfortable to sit up straight than to slouch... that's the posture she's using. I think the fact that she is eating and pooping is a good sign for the digestive system though. Is the poo fairly normal? Her weight normal? No signs of worms?
I haven't encountered this issue personally but it looks like there are a few threads out there on it. There's some interesting info on this thread:
https://www.backyardchickens.com/t/4667/chicken-walks-funny-holds-tail-down-doesnt-eat
Does her crop feel normal?
If the digestive stuff seems OK and she is eating OK, I would be very suspicious of internal reproductive tract issues- especially if she has stopped laying and there doesn't seem to be an egg stuck (keep in mind though, any medical or other stress can cause them to stop laying). I have not personally dealt with a hen who is "internally laying" but that might be a good search term to use on the forum? Or, maybe she just has an ouchy infection and needs some penicillin?
It's normal for their temperature to be warmer than with a human (I think around 103F is normal). You can check pretty easily with a medical type rectal thermometer. They can run fevers, but I'd think she'd be a bit more fluffed up and miserable looking if she was that sick.
I wouldn't be too shy about feeling around for an egg (as far in as necessary)- just be gentle/don't force anything... ya know, common sense
. You might be able to palpate gently for an egg externally too- might be able to feel it if it's low in her reproductive tract.
I hope this is some help at least- and that it resolves on its own. She's a pretty girl
