Not a 100% indicator - just would tell me to watch closely.
Have you read the thread about lethargic duck?
https://www.backyardchickens.com/t/833809/urgent-soft-shelled-eggs-and-a-lethargic-duck
It is a long read - but the highlights are that PetDuck's Quackers had some classic EYP (egg yolk peritonitis) symptoms, it turned out to be the diagnosis, and she got help with antibiotics and needed to tube feed Quackers till she bounded back. And Quackers recovered.
Read the very first post in that thread for what it looked like.
Symptoms that come to my mind (I have lost two ducks)
lethargy
slightly squishy-feeling lower abdomen
may or may not lay odd eggs frequently
far-away look in the eye
weak
loss of appetite
stumbling
In Shelly's case, she seemed off, went to the vet, nothing appeared unusual, and six weeks later she died quietly after refusing to come out of the night shelter.
Although some vets are pretty fatalistic about it, I believe that in some number of cases, antibiotic treatment and support with vitamins and diet and TLC will pull them through.
Flip side of that is that a friend with many pekins says that in her flock, the soft egg layers are not the ones getting peritonitis for the most part.
I have to be away most of today - - - but if you can, get a vet to look at the duck. Feel her tummy gently to see if it is squishy. Give her some poultry vitamins in her water once a week for now. Does she have any of those other symptoms?
I think the medicine they gave Quackers was Baytril - prescription only. And it kills the duck's appetite, often. So probiotics and possibly tube feeding, worst case.