We brought her inside by herself yesterday evening, and the first thing she did was eat lots, and as I checked on her throughout the night, she seemed to be asleep the whole time. She does seem rather thin compared to the others, so I'm guessing she just wasn't eating as much as she should have. She still looks depressed, though, and is all puffed up.
I'll see how she does over the week, and then consider the next step.
Thanks for all your concern!