Good pics. She doesn't appear sick, and the poop is normal. Your guess that this may be related to bullying could be the right one. Here's something you can do to cinch that guess.
Take away all the food from your flock for several hours and then return it. Observe. No doubt this hen will head to the feeder with the others. Watch what they do to her and how she reacts. If they peck her and she almost immediately gives up and wanders away to peck at the ground away from the others, then you have yourself a bully victim who is likely starving.
Right now, give her a cup of warm water with a teaspoon of brown sugar or white, stirred in. This will elevate her glucose and she should then perk up considerably. Then you should find a place for her to eat in peace without competition from the others.
But she would need further help to restore her self confidence and give her the courage to compete for food. This will explain how to do that.
https://www.backyardchickens.com/articles/chicken-bully-chicken-victim-a-two-sided-issue.73923/