She could have a sour crop or an intestinal obstruction. If her crop can't empty due to a blockage, the food inside starts to spoil and the yellowish droppings indicate that she is producing bile for digestion but has insufficient mass of food going through her to absorb the bile. Is it possible to take her to a vet? I had a similar experience with a buff orphington hen. She had swallowed some staples which blocked her intestines and she displayed almost identical symptoms. The stapes were discovered by x-rays and she was scheduled for surgery but, unfortunately, died before it could be accomplished. I hope this isn't your problem but a trip to an avian vet would probably be in order.