Both my ducks are female, the other is a chocolate Indian runner. She is a year older than This duck, laying fine. This morning her poop looks normal, she does not seem weak, holds head up, eating and drinking. Just not herself, tail very much curved down, in the bathtub with water she looks the most comfortable, she will stand, do ok if she doesn't try to move, but no walking yet this morning, though she was waddling last night in a weird way. This morning when she tried she ends up with both wings out like she can't keep her balance and then just lays down. She's soaking now, but I'll try to get a video in a while when we drain the water.
Duck with tail tucked down is absolutely a sign of something very wrong... no healthy, fine duck will tuck it's tail down and keep it like that... waddling excessively when they normally don't is abdominal issue, usually associated with a stuck egg... like Kathy said, it could be a shell-less egg or up to high to feel...
Try getting some more calcium and vitamin B into her.
X2
I would get the calcium citrate or calcium gluconate asap into her, warm soaks and see if she passes one...