Duck with drooping neck and balance issues

I have a crested Pekin duck that has had some issues in the past where she would try to stand up, drop her head and neck down low and stumble backwards. The first time she did this (about a year ago), I thought she had broken her leg or hip. I brought her in, kept her quiet and still and before long, she was up and running again. Then recently it happened again with the same results. But today, I literally thought her neck was broken. She would try to stand up but her head would drag the ground. I picked her up and her head just drooped. I held it up and brought her in, wrapped her in a towel and just watched her for a little while. After about 30 minutes, she just stood up, flapped her wings and looked at me as if to say, "it's time to take me back out!" I took her out and watched her for a while. She acted like nothing had happened. She walked around with the other ducks, ate some food, drank some water so I thought all was well. This evening, I checked on her. She was walking around just fine. Then all the sudden, she just laid down and couldn't hold her head up. As I went to get her, one of the other ducks actually walked right over her, but she didn't even try to get up.
There is no rotting food around and the water supply is completely fresh all the time because I have a pump in the creek which pumps the water up to a garden pond which spills into another pond and then flows back to the creek. Lately, it has rained a lot so their area is pretty muddy. I normally keep hay around on the ground, but I didn't have any the other day when the rain started. I would think it would take more than just a few days of wet weather to cause problems. I've read all about botulism poisoning, but I really don't think this is the case since the other ducks are fine and this has happened with her before. My wife wonders if she's having seizures, but it doesn't really seem like a seizure either. Tonight, I noticed one side of her head (from the bill to just past the eye) is swollen and her eye on that side seems irritated. The other eye is perfect. I thought this could be from her head being walked on when she couldn't get up, but just not sure. She is 3 years old. Any help will be most appreciated.
And in case anyone is wondering about my username, yes, I have a 3-legged duck! More on that later. For now, I want to help my little friend.