Well, the vet had me cut the dose to 0.25 ml every 12 hours (as opposed to 1 ml), and he said if there is no improvement within 48 hours to give him a call and he will give her a different antibiotic. He's concerned it's become resistant. Guess we'll see...
Also, he said that technically, the dose the ER vet gave wasn't wrong, it's the maximum recommended dose for a mammal of that size, but of course, chins' guts are much more sensitive than many other mammals, which is why she had such a problem.