From your description she sounds exactly like a broody or pre broody duck. My welsh harlequin behaved exactly like that for several weeks last year. I didn't want her to be broody because running around quacking angrily takes a lot of energy, and I wanted her to relax and eat. So I took away nesting boxes, destroyed her nests daily, removed eggs promptly (and egg shaped rocks!) and took away any items that created dark cozy cubby holes.
Here's a couple videos of my duck, Pigweed, when she was "peribroody" last year. She usually comes right to me when I call. Sorry they aren't so great and one is sideways....
You can always weigh her regularly. I do it daily for my ducks when they are sick, and every few months when they are not. Assessing weight visually isn't the best method, palpating the keel bone and having a weight history is preferred. I like to use a baby scale because my ducks seem most comfortable that way, but most members just use a kitchen gram scale and that works fine!
A vet can determine the best course of action for you. I'm paranoid and take my ducks to the vet when they stub their toes, but I didn't take my broody duck in. Something different could be going on with your sweetheart, though.
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