Take a fecal sample to the vet. Test a few samples in the course of a week to be sure. Then you will know if there are any parasitical infestations.
I would avoid blood work right now, because that would be a bit traumatic.
Make sure she is in a quiet room with a few hides, by herself. Keep a LOG! - See how much she will eat today or tomorrow (leave a full bowl of mealworms in with her and throw a cricket in to see if she'll eat). Write that down each day. See if/when she is using her toilet corner and note the condition of the poop (runny, solid, undigested prey, etc)
She's most likely stressed out from being sick/mistreated so she might not eat right away. If she's eating at all, that's awesome! I have had several resuces from my vet office & the Humane Society that I had to syringe feed food and medication multiple times a day for 2 weeks to over a month.
Make sure she's getting vitamins. Stick a calcium dish in there with her too. Keep the warm side about 90 deg.F.
Sorry if some of this is totally rhetorical and obvious!
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Too many Leos
1.0 feline "Spot"
0.1 canine "Tika"