See what you can do about being able to handle her so that you can get a closer look at her. Since it is warm where you are, get a tub of some sort - kiddie pool, concrete mixing pan, small water trough - really large bucket. Something.
Get her into water so that you can watch her closely for a while. Look for anything off. Swelling, tenderness, unusual heat in her legs or bill.
It is not always easy - but if you cannot handle your duck, it will be difficult to find what is wrong.
She may have a systemic infection and need antibiotics right now.