I would start treating her for coccidiosis, since that's my first thought. Has her poop been pretty normal? Even if it has, treating for coccidiosis won't hurt. Get amprolium, usually sold under the brand name Corid. It comes in either a liquid or a powder, either is fine. You will need to add this to her water, and to the water of the other chicks as well.
Follow dosing instructions in this article:
https://www.backyardchickens.com/ar...ol-ampromed-the-correct-amprolium-dose.73341/
Just in case it's not coccidiosis, can you give us a little more info like what you are feeding them, what type of bedding you're using, etc?