Prayers for Taz.
Let's start with some ideas. The antibiotic could cause the diarrhea. I would stop the antibiotic and take her to her regular vet.
FORCE feed her if you have too. Once she gets something in her stomach she can start to feel better. It is like a snowball effect. The cat doesn't eat because it doesn't feel well, and then it feels worse because it didn't eat. Often you need to force feed to get them back in the habit of eating (I know it sounds weird, but it's true). If nothing else, get some Karo syruo and rub it on the gums. Get's the blood sugar level up.
I am not sure what "all the tests" included, but Taz should at least visit with her own vet, not just the ER vet that may have never seen her before.
More prayers.
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