I agree, that the likely cause is you applying pressure to her crop area. When you apply pressure to her crop you squeezing the food/water back up her throat, it's very important you avoid doing this right after they eat, as especially with ducklings, they aspirate very easily.
As far as the sand question, the answer is no, it's way too fine and can cause a blockage in her digestive system.
As far as the sand question, the answer is no, it's way too fine and can cause a blockage in her digestive system.
