Chickens hide pain. Have you examined the bottoms of her feet for bumblefoot? Could she have jumped from too high a distance, twisted an ankle, bruised her foot, broken a bone? Could be a slipped tendon. Does she have pasty butt? Usually pasty butt is reserved for younger chicks, but an overheated or sick chick could end up with this problem as well. How does her crop feel in the morning? How does it feel at bedtime? Should be empty in the morning and full at bedtime. Splayed legs could indicate a neurological condition, or malnutrition (typically malnutrition of a chick starts in the egg and manifests itself in the chick; a malnourished hen cannot put into the egg vitamins/minerals/amino acids that she's lacking). What does her poop look like? Can you take pictures of her, can you take pictures of her poop? Do/Have these chicks had access to soil, grass, the big outdoors? Have they been wormed? Does their food contain amprolium (coccida inhibitor)?
You need to thoroughly examine the chick from one end to the other. Post your findings here so that folks with more experience can GUESStimate the likely cause of her distress.