You said that your hen's crop was empty. Usually with sour crop, the crop is full, balloon like and the chicken may spit up foul smelling brown liquid.
How old is she? Has she had any laying problems in the past? I would probably get some Sulmet or Dimethox (sulfadimethoxine) at your feed store, and treat her for a possible intestinal infection. It treats coccidiosis and some other infections. Corid also treats coccidiosis, if you have that...
Are you seeing any diarrhea with mucus or blood in her droppings? How old is she? Coccidiosis can be a problem with younger chickens, or with ones who have moved to new soil, or when new chickens have been added. Corid is the treatment in the water. Look her over for lice and mites. If she...