Could be sour crop.
Sour Crop
This condition can develop when food remains in the crop or gullet for more than 24 hours due to a blockage, and begins to ferment and go rancid. Your bird may have a large, squishy feeling crop with a bonus of bad breath. You might be able to smell it just by holding your chicken, or try to open the beak and take a sniff; you'll know right away. Same remedies apply, massage and some oil. You can also try to turn your chicken upside down and rub the crop in the direction of the mouth to get them to vomit up the contents. This is best done under the supervision of a veterinarian to help prevent the contents from entering the lungs.
Gently massage the crop with your fingers. This can help loosen up the food and help it move down the digestive tract. Olive oil can help move things along as well. Use a few teaspoons in an eye dropper and carefully insert it about an inch or two down the bird's throat. Enlist a friend, as you don't want to push the oil into the lungs if the bird struggles. Massage the crop to distribute the oil, and wait until the next morning to see if the crop empties. If it's still very hard or your chicken is very listless or losing weight, a veterinarian can surgically remove the contents. Offer water only; chickens can live about a week without food.