It isn't necessarily something you have done, she may have brought something with her and it has developed to the point that it is now bringing her down. My first thought here is worms or maybe Coccidiosis, It is recommended to keep chicks off the ground for twelve weeks and fed medicated feed for that entire time to help boost their immune system. Since you have other birds on the property either of those two things may be in her system.
I have two broody raised chicks left, the third died from worms and had the same symptoms you are describing. Read the thread 'Safeguard and Feather Damage while Molting' for more details and dosing. You can also dose with Corid at the same time as dosing with Safeguard. If the chick can not eat the meds can be drizzled down the throat by syringe without needle. Also read the thread about Corid near the top of the forum list started by CaSportPony.
I have two broody raised chicks left, the third died from worms and had the same symptoms you are describing. Read the thread 'Safeguard and Feather Damage while Molting' for more details and dosing. You can also dose with Corid at the same time as dosing with Safeguard. If the chick can not eat the meds can be drizzled down the throat by syringe without needle. Also read the thread about Corid near the top of the forum list started by CaSportPony.