I truly have no idea what really killed her. The other two are growing and doing great! After further research, I think it was duck virus hepatitis. But I'm not positive. I found one of my year old barred rocks dead inside their coop yesterday morning.
I've heard and read you usually will lose at least one, and fortunately, so far I haven't lost any more than that every time I get a few. This is also my first year doing ducklings. We got 17 chickens last year, and ducks are a lot more work that chicks, that's for sure! I will have to pick up some Nutri-Drench this weekend, thank you for all the help, y'all.