I am wondering if she has a back injury. Remember, ducks can recover from awful injuries - so don't see this as necessarily insurmountable.
The fact that she's acting like she wants someone to mount her, perhaps (just based on your description) makes me consider a hormone imbalance.
Just thinking out loud, but I would try some gentle massage. Just pet her back, from the shoulders to the tail. I do this for ducks who are having egg difficulties - get them in a tub of lukewarm water, and while they float, just lightly pet their backs. This means usually that they swim against the direction I am petting them, and so it gives them a little exercise as well. I think it improves circulation. And touch can be healing. There are horse people who bring in someone trained in Reiki or massage when their horses are injured. It might work for something like this.
And though we cannot always know . . . please try to think back - what changed? Even something seemingly unrelated may hold a clue.
Could she have been bitten, or stung?
There can be infections that get into the spinal fluid. There can be toxins in the environment . . . has anyone been spraying around your place - your neighbors?
