At that age, I would suspect coccidiosis, and buy some Corid (amprollium,) and treat the water for 5 days. Symptoms of coccidiosis are lethargy, hunched or puffed up posture, diarrhea or blood in stools, and refusal to eat. Treat all chicks at the same time. Dosage is 2 tsp of the liquid Corid, or 1.5 tsp of the powder per gallon of water for 5 days.
I'm sorry for your loss. She may have had some problem with her immunity, but keep the droppings stirred in your bedding, and try to keep it as clean and dry as possible. Make sure they take the Corid. Cocci is in the dirt and they need to get exposed to it to develop immunity, so hopefully they will start to build up resistance to it, and you will not lose any more.