Miss Lydia has offered some good points, here.
I agree we need to think about as many possible causes as we can - heat, stress, infection, nutrition.
By the way, there is some variety in optimal nutrition among ducks - we need to try things and see what works best for those who seem to have more need of some vitamins and minerals than are in standard food.
If she has an infection, it could be in her digestive tract, egg-laying system, or her entire system. That last one is called peritonitis and can be caused by egg yolk getting loose in the body cavity. In any of these cases, she needs oral or injectable antibiotics. There is something here called Baytril, it requires a prescription, but it has been used for peritonitis.
@casportpony and @Lacrystol may have some suggestions on antibiotics - there are also some threads you can search for.
We are not vets, but a number of us have to be our own duck health care providers for different reasons. Some antibiotics are available here at feed stores.
If it's heat, like Miss Lydia wrote, cool water, shade, some relief from the heat.
For nutrition - calcium and phosphorus need to be balanced, there needs to be vitamin D in the diet as well for the Ca and P to be optimized.
I have used calcium gluconate, available here as a 23% solution at a dosage of 1 teaspoon per half cup of water, added to food to make it oatmeal consistency (it's the best way I could come up with for increasing their daily calcium intake). Some of my ducks will eat crushed oyster shell free choice. Some will not.
Since I have ducks who tend to lay soft shells (4 of 10), the Ca gluconate was getting a little expensive so I switched a few months ago to dissolving calcium citrate tablets in water, and adding between 50 and 80 milligrams of Ca citrate per duck (that's about 500 to 800 per day for the flock of ten).
And I am seeing some improvement. We still get one or two soft eggs a week instead of 6 or so. And, we still get those soft ones. So I am still working on helping every duck in the flock get what she needs.
I have also added Ca gluconate or Ca citrate solution to a couple tablespoons of mashed peas to individual ducks who are having troubles.