From what you have described, I myself, think that the other chick you had put with them was picking on Omlet during the night, or at times you didn't see. I had a young roo that was acting strangely---sitting in the same spot all day long by himself and sleeping, not wanting to go in the coop at night. I thought maybe he didn't feel well, but just kept watching him for any other signs of illness. After about 3 days of this, I had gone out to put the chickens up for the night, and found him dead just inside the door of the goat barn. I believe that the oldest rooster I have actually "bullied" him to death. I think he was getting picked on at night after I put them all in the coop----that's why he sat and slept all day long. No other chickens exhibited this behavior or got sick. So, hopefully that is the main problem your chick is having at this time, and she will get better quickly now that the older chick is back in the other pen.