Welcome
I'd like to know what this duckling is being kept in, pictures would be great. Mild stages of Pododermatitis are already evident on the bottom of her feet which is a good sign that the conditions that she is being kept in are not optimal at all. As far as the swelling, she seems too old to be considering a yolk sac infection, so I might guess ascites, how does it feel? Another thing to consider would be a ruptured air sac, here is some more info,
https://www.backyardchickens.com/threads/subcutaneous-emphysema-pictures-added.828000/
Nonetheless, this is not normal and she should be taken to a vet if possible.