Ducks can do some things that are impossible to understand. My priority is to protect them all - and separating was the right thing to do. Make sure she doesn't have any hidden injuries and if she does, get them treated.
I would get her into a tub of lukewarm water and look her over really really well. She may have an infection, parasites, egg yolk peritonitis, or something.
Longer term, sometimes just splitting the flock up a few different ways over several weeks can change their behavior.
Ok thank you! I'll check her in the morning. It's really dark out now!