I agree. My next question is, have you wormed her, and with what? Do you see any rice looking bits in her poop?
Since addressing lice/mites and worms are things we can easily try, that would be my next step, if your birds haven't been wormed. Of course there could be a number of things going on. I'd guess she is low in the pecking order as well, and perhaps not being allowed near the food enough, except she didn't eat when isolated. The "fluffed up and looking cold" picture is typical of cocci, too; are you ion a very wet climate? Adults do get sick with cocci, though it's not nearly as common or likely as in young chicks.
Does she seem thin or underweight? You can compare her "feel" to another of the same breed, or see if her keel (breast) bone is more prominent, for a guess. If you have a scale that would work, you could start weighing her daily at the same time of the day.
While she may simply be more sensitive to the cold than the others, her failing to eat when warmed makes me think further investigation is a bood idea.
Lastly, does her belly or underside seem at all swollen or enlarged?