The dosage of metronidazole is 5.5-11.5mg per pound. At maximum this kitten could have 16mg of metronidazole which is an antibiotic/antidiarrheal/anti inflammatory for the gut. Yes, it is used to treat diarrhea and giardia. It is unlikely you have a pill small enough to even quarter to get the dose you need. If you do happen to have a 50mg tablet then she could have a quarter of a tablet.
Diarrhea with blood can be consistent with feline distemper or intestinal parasites in particular coccidia. It is possible that the amoxicillin is causing some issues as well. If you are able to dose with the metronidazole then I would stop the amoxicillin until you can talk to your vet tomorrow. I would still consider halting the amoxi no matter what but contact your vet first thing. At least this will give your kittens tummy a chance to settle down if it is related to the amoxicillin. I would be sure to take a stool sample into your vet to evaluate for intestinal parasites.
http://www.critterchat.net/giardia_fands.htm Take a look at the link below for dosage information.
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Then yes, she would receive 1/4 cc of that dose.
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