Good advice so far, I would bring her in somewhere clean and cozy where I could watch her closely. I would give her a lukewarm bath deep enough to float in, since that helps me really take a good look at a duck. When Romy is not feeling well, at first she will just float, stationary, silently. As she begins to feel better, she will begin to paddle and occasionally make a quiet sound. When she is much better, she is paddling, wakking, and splashing.
So something nasty could have been eaten, there could be some impaction (some ducks will eat strings and such, which stop them up, and that can be serious)... There is the possibility, perhaps of worms, another serious thing.
In any case, I would give vitamins with electrolytes and especially probiotics in water while sorting out what next, looking for a vet or such. A higher quality feed would be helpful to her also, now and long-term.
Thank you for sharing your concern with us on the Duck Forum, please keep us posted.