Hi - and welcome to YT
. So sorry your little one is sick. Something is most def going on here.
The fact she is hiding and isolating is very concerning here. But how is she staying outside all night...? I'd put a fast stop to that bc of predators in your area...plus it's too cold at night for her.
If I were you, I'd get to a Vet Internist asap rather than continuing to play guesswork with a generalist. I so hope you're able to find out what's wrong soon. Poor kiddo.