Sounds like it could be a neurological issues which is sometimes from a vitamin B3[niacin] deficiency you should start your duck on 200-250 Mg's of Plain niacin asap, if you can find it getting harder all the time to find plain, might check amazon if you have a hard time, or Nutritional yeast[natural food store] niacin tablets can be crushed and added to 1 gal of water then offered in waterers, or sprinkle the yeast on top of their feed, but your duck need some right away even a complex B vitamin would work. as long as it has Niacin in it. and please don't leave it in the water un supervised. Duckling can become lame and develop neurological problems from not getting adequate amounts of niacin some seem to need it more than others and using a feed mainly developed for chickens doesn't have the right amount. so we have to supplement. Please let us know how your duck is doing usually once you start on supplementation it begins to correct itself in a s little as 2-3 days.
Could very well be , might want to talk to where ever you get your feed has it got an off smell? if you think it may have eaten moldy feed there are flushes you can try also
