I am so very sorry to hear this. I understand your decision - it was very brave. I have had this happen before and sometimes we just don't know. This same situation happened with my Silver Laced Wyandotte and I have no idea why. She just kept going downhill and nothing I did for her worked...
If it is respiratory she will need antibiotic as soon as possible (the kind that you mix in water) about 30cc I believe ) such as Duramycin. See if you can get her to eat any yogurt with live cultures and scrambled egg for energy. Good luck!