Does the poop look like it has mucous?
A photo of the legs may be helpful.
Make sure your brooder is not too hot - only have ONE warm spot and let the rest of the brooder be as cool as you can get it. Place the food and water on the cool side of the brooder.
See that there is plenty of ventilation - if she is in a rubber tub make sure it's not too hot and has sufficient air flow.
Try to keep her hydrated. Wetting the chick starter may help as well.
Have their been other chicks in this brooder? It may be wise to treat for Cocci, she looks quite fluffed. It could be that she's too hot, but it could also be Cocci since you report her lethargic and laying about. Treatment is with Corid which can be found at most feed stores.
Corid dosage is 1 1/2 teaspoons Corid powder per gallon or 2 teaspoons of 9.6% Corid liquid per gallon. Give for 5-7 days - make sure this is the ONLY water available during that time period. Mix a fresh batch at least once a day.
Let us know how she is doing.