Thank you, all of you. She was not free-ranged, she was kept in a large 33’ by 33’ pen with 9 other chickens. I fed them all scratch grains. This one (along with 2 others) was moved over here one week and 3 days ago from my uncle’s. She has more room, and far more ventilation, then she had over there, so she shouldn’t have had much issue... but she had been pecked at terribly around her comb 3 days prior to getting sick, so maybe that stress made her susceptible to a problem?
My uncle also fed them laying mash instead of scratch grains, so one thing I should have done is switch over to the laying mash so that was one less stresser on their system (new food), and gradually changed the ratio until they were eating the same as my current flock.
What’s weird is, the 2 other Plymouth rocks I got from him aren’t showing any signs of diarrhea or anything like that. It’s some of my own hens who are. So...

It makes me think it was a virus his birds weren’t used to, or something like that. Because if it was the food, my own birds would have become sick, wouldn’t they? And the 2 other Plymouths would have gotten sick first. But my birds were fine when I switched their food over.
I know there are a lot of folks that say you shouldn’t feed them more than 15% scratch grains, or scraps, or anything like that, but I know folks who only feed their birds whole sunflowers, kernels of corn, oats, and something else, with kelp mixed in for vitimins and minerals, and they THRIVE, so I thought it wouldn’t be a big deal if I did.
I don’t feed the minerals though... I need to get the minerals. Anyways, I’m switching back to layer crumbles until everyone feels better, then I’ll see what I should do from there.