Goosebaby
Free Ranging
Extra fluid in the abdominal cavity “ascites” which causes their abdomen to enlarge and sag down can be due to heart failure, an organ issue, or issues with the reproductive system in female birds, or even a tumor.This is her, she is a Buff Duck and I don’t think she stands abnormally tall? Her feathers have been fluffed all day, it’s in the 40 Fahrenheit so I don’t think it’s because of weather?
If that’s what’s been going on with her the last few years I doubt it’s heart related, that progresses fairly quickly so it would point to an organ issue, or possibly reproductive, probably unlikely because those are progressive.
She looks fine from that picture but that isn’t a good judge, if you think her abdomen is abnormal it probably is.
When was the last time she laid an egg?
Her other symptoms, the watery and green droppings are from her not eating.
The tail twitching could be from her straining to pass an egg or she has an internal mass, possibly she is constipated as other users have mentioned.
It’s really hard to say because there could be any number of things causing this but considering that you were giving calcium supplements she could have hypercalcemia which can cause constipation, muscle and bone weakness, kidney issues and as it progresses heart and nervous brain issues.
If that’s what’s going on you need to flush her system, get her to drink as many fluids as possible, maybe try an oral flush/laxative like molasses.
https://www.backyardchickens.com/threads/flushes-for-aiding-in-toxin-removal.700526/