Since they can get picked on if separate more than a few days, I would let her be with her flock some, so they all stay familiar. I usually keep sick ones with their flock unless they are pecked or bullied. Chickens can feel puny at times, and checking her skin below her vent and elsewhere for mites or lice, consider worming, and offering some egg or tuna, and mushy wet feed to give her energy is how I treat mine. What does her poop look like? Check her crop first thing in the morning to make sure it is empty.