All kinds of things, honestly. Reproductive disorders, infections, worm overloads, diseases.... There are so many things that.can go wrong with chickens. That's why I suggest keeping an eye on her and taking note of her crop function, weight, body condition, and behavior for a while. If...
If she has food in her crop at night, then she's eating. She sounds like she's doing ok. You might weigh her to see if she's gaining at all as she recovers. You can also feel for her keel bone. If it's very prominent and sticks out a lot/feels sharp, she's underweight. She may be underweight...