Sometimes, for different reasons, the egg-making system gets stuck. Then an egg that should be passed stays, and that can cause infections and a pileup of eggs sometimes. Sometimes the duck will strain to lay until she pushes part of the tube that carries the egg outside her body - a prolapse.
The tub time can help relieve discomfort. The calcium may help because between the two, if she is egg bound, that could get the egg out of there naturally.
Make sure she has plenty of fresh water and good food.
Hennible's vet said that advil is okay for ducks.
For ducks 6-8 lbs 25-40mg twice daily adult tablets cut up or children's liquid Advil 1.5ml twice daily, orally.
Amiga’s arithmetic – 8 pounds 40 mg is 5 mg per pound
6 pounds 25 mg, is just over 4 mg per pound
so, 4 to 5 mg per pound of duck - you need to know how much she weighs.