She seems awful young to have that. Have you treated for mites just in case? They can do that to the rear-end area. I'd dust her with permethrin powder and the coop too.Everything I’ve researched says aged layers. She’s only 9 months
Plucking by other chickens can also do that. Any bullying noticed?
Has she laid an egg recently? If not, I'd give her a calcium pill, as in open her beak and pop one of yours in, around 400 mg. If it has anything to do with her reproductive tract, the calcium will cause contractions and she should expel it.
Not saying it's not ascites, but I'd try a few other things first before culling her.
