Roos

Note the time it appeared. Right after eating or out of nowhere is most of the answer.

Right after the feeder, soft, and a duckling behaving normally is almost always food. Ducks do not have a true crop like a chicken but the oesophagus is expandable, and a duckling that has been hungry and then gobbled packs it visibly into the neck.

The test is time. A food bulge feels soft and doughy, maybe a bit lumpy, and goes down over the next few hours as it moves along. Check she has water, because they need it to swallow properly, and look again before bed.