Hold on! Before you spend money on a vet - and it may be a LOT of money, use some common sense:
Is she eating, drinking pooping waddling around with her flock mates, quacking reacting to you?
If yes, she is fine. Ducks are very good in hiding sicknesses and injuries from the rest of the world, but if you know what to look for, they can't fool you:
A sudden change in the duck's personality is an ominous sign. Thursday i noticed that Katharina Duck wasn't storming out of the house in the morning, quacking loudly, bobbing her head and pushing the other ducks around to get to the cat-food. Instead she just walked out, said nothing and waited patiently for her turn. In the afternoon she kept separate from her flock-mates and was sitting down a lot, very quiet and her lower abdomen looked swollen. I was worried, but she was still walking around, went to bed on her own legs so i decided to give her another night. Next morning there was a humongous triple yolk egg in one nest and Katharina was back to her normal cheekiness.
So if your duck doesn't limp, does not stay separate from her flock and does not sit down frequently - don't worry so much. Just be an attentive observer.