Yes you would definitely note a prolapse of the oviduct.
It's still so hard to tell what that is - I don't have ducks, but I do wonder if it's damage from mating since she was roughed up (maybe the male didn't withdraw right- is that feasible

- chickens only "kiss" vents, so there's no penetration like with ducks)
It could be a papilloma or possible a cloacal prolapse, but if it's staying in then I would just watch it. The salt baths can be soothing to tissue like that.
@Eggcessive might have a better idea.
Can you get someone to take photos while you hold her so we can see it better?
I am glad to hear that she was quacking at you and that she laid an egg with no problems though.