I wouldn't worry about it unless she's having other symptoms. My ducks had some obvious other symptoms.
And you have the benefit of her being at the vet before. They've surely palpated her abdomen. My vet always does and can tell if it is reproductive inflammation or not.
Sorry I've worried...
It's probably not an issue. When it has been for me my duck was also experiencing diarrhea, or in another case lethargy. Usually it is just normal reproductive activity.
In my experience sometimes that, "under belly," can be inflammation due to reproductive activity. It can be completely normal. With other symptoms it could indicate a problem.
It is really challenging to assess avian weight condition by observing birds. Even avian veterinarians feel the breast on either side of the keel bone before determining body score - on all birds, not just ducks. Pekins are particularly fluffy and waddly, in my experience, even when they are...