In my experience with ducks, the two most prominent causes of hard-like abdominal distention are egg binding and salpingitis. Given her age, I would find the latter to be less likely since it is a reproductive disorder that tends to develop as the bird is in its older years. Abdominal distention due to a reproductive disorder or tumor isn't likely to occur in such a short period either.
Depending on how far the potential egg is in the oviduct tract, it may or may not restrict fecal matter from flowing. If you cannot take her to a vet, I would probably suggest keeping her in a warm, quiet area for now, and consider administering a calcium supplement to help induce passage of the possible egg. Try to keep her hydrated, by offering her electrolytes often throughout the day.
I hope you see improvement soon.