I feel like I have seen this before . . . . drat!
Okay, is it warm? Can you very gently touch it, does it feel smooth, squishy or lumpy?
If you have a vet, I would take the duckling in.
In the meantime, can you give the little one a nice lukewarm bath deep enough to float in for a while, and watch carefully?
I am wondering if there could be some impaction, causing food to back up in the tummy or lower throat area.
Also, might there be internal parasites, like worms?
Aside from that, could the duckling have gotten into something, like a ball of twine?
Had the food bowl become empty and when it was refilled the duckling gorged itself?