Lethargic, yellow-pooping hen (I have poop pictures!)

This morning when I went to open the coop, Minerva (Rhode Island Red, quite young, has only been laying for a couple of months) was just quietly sitting/lying in the coop and didn't rush out like normal.

I've isolated her from the others, and while in her isolation pen she mostly is just sitting/lying down. If I stand her up, she sometimes stays standing and will walk around and peck the ground. She's also doing strange watery yellow, and yellow and green poos (the photos below are all poos that she's done today). Her feet also seem a bit more scaley (the scales are slightly raised) than usual. When she's sitting, her tail points downward. Also, sometimes one of her eyes is closed or partially closed while the other is open. She's been drinking a little water, but not eating much.

All my other chickens are acting normally.

I've read some of the other threads from people having similar problems, but couldn't find one that had a diagnosis or a solution that had worked for anyone.

Following some advice from books and online, I've given her some probiotic yoghurt (she's only had a tiny bit, I can't get her to eat it) and some gatorade (again, she's only had a tiny bit, seems uninterested in it).

Not sure what to do next or how to help her?!

She just laid an egg, and it had these weird scratches on it - I've never seen that before, no idea if it's relevant