Update, after going back to the vet a couple more times this week (I work there, so it's just a matter of bringing her in with me, thankfully), Sheba does need a stronger dose of her diuretic. And a little stronger appetite stimulant.
Twice now she's gotten sick while being treated
for UTIs. She didn't show any sign of being sick while she had the UTIs. The first time, a year and a half ago, it was just a routine urinalysis as a precursor to bloodwork, and she didn't show any signs of having a UTI or being sick at all, until she was on the antibiotic. This time, I just happened to notice that her urine looked a little pinkish. Again, she was acting fine until she was on the medicine. Both times, she went off her food and acted depressed, withdrawn and moody while on an antibiotic.
But now she's eating better. Still very weak, but acting stronger and more herself. Yay, Sheba!