I have a 3 month old Australorp that is lethargic, has a droopy tail, and isn't eating. We have isolated her and she has access to food and water.
Get her to drink some chicken vitamins with electrolytes in her water. Most feed stores sell SaveAChick, Durvet, or Poultry NutriDrench. At that age, coccidiosis as well a s dehydration could be a problem. Symptoms of coccidiosis are lethargy, hunched posture, not eating, and diarrhea with mucus or blood in droppings. Corid or amprollium in the water is the best treatment for 5 days.