Not sure i can add much, but ask more questions. Are both ducks female? If so, is the other one laying yet? If the other is male, does he mate her? Obsessively?
Since they free range, I would guess it is possible for her to have gotten into something she shouldn't have. I believe watching her for a while, and caring for her inside will hopefully give you more clues. The Tums are a good idea, if she is egg bound. Also try massaging her belly, stroking toward the vent.
As far as poop, did you take a picture? Duck poop can be all sorts of alarming, depending on what they get into. May be bad, may be nothing to be alarmed about.
Let's enlist advice from
@Miss Lydia also