How old is she? Do her droppings look as if the food is not totally digested? That can be a sign of necrotic enteritis, which can affect chickens who have earlier had coccidiosis. It is usually caused by a bacteria C.perfringens in the soil, but other organisms can cause it. If possible, I would take in several fresh samples of this chick's droppings to your local vet to test for coccidia, bacteria, and worms. However, if you can't do that, then I would treat with Corid for cocciosis. You can start the treatment with Corid, and still take in a fecal sample. Dosage of Corid is 2 tsp of the liquid, or 1.5 tsp of the powder, per gallon of water for 5-7 days. Enteritis can be treated with many different antibiotics, but a test would narrow down the right ones. If she is pretty young--under 12 weeks, then definitely treat for coccidiosis first. Here is some infor on cocci and enteritis:
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html
http://www.thepoultrysite.com/diseaseinfo/101/necrotic-enteritis/