Ascites can be caused by ovarian problems in hens, other possibilities can be heart, liver or kidney problems.
It is possible that it could be a bacterial infection of the perineum, but usually that’s secondary to the other problems.
Usually these problems don’t come and go, so the fact that she got better makes me wonder if she could have pancreatic issues.
My gander Parsnip developed weight loss and was refusing to eat in the fall/winter of 2019, his tests came back normal for everything other than his blood amylase was through the roof. He had a weirdl swollen abdomen, not to the extreme, but she said it felt like a female about to lay, so his vet sent him home with meloxicam, that was really all she could do.
If you try to look up pancreatic problems in poultry you aren’t going to get much, it isn’t studied in “poultry,” and I’ve only found a few references in parrot forums and articles, so it isn’t hardly studied in avian medicine at all. I was basically flying blind with Parsnip’s treatment.
All I could do is give him meloxicam, turn his feed into mash to aid digestion and encourage him to eat, and I gave him as many blueberries he wanted, which was a lot because it was the only thing he wanted to eat.
His mild “ascites” returned several times coupled with his bouts of not eating, I learned that it helps feeding him only greens and watery fruits and vegetables during these bouts, tomatoes and berries seemed to have had a strong effect. I also gave him b vitamins, gave him milk thistle, aloe detox, grapefruit, and tart cherry extract “it like the grapefruit has a calming effect on the pancreas” during his episodes.
So far he hasn’t had an episode in the longest stretch so far, since he molted last year.
There isn’t a lot to go by with information about pancreatic health in birds so not every case may present the same, if nothing else you might consider getting a CBC to see if her amylase is elevated.