I agree with Angie, if your vet has not done this then you need to have a fecal PCR test done to make sure that the cause of the diarrhea was correctly diagnosed and treated. So you need to ask them what test they did when they diagnosed the Giardia, and then find out what you treated your kitten with too.
And agreed that a probiotic is a good idea, kittens' gut bacteria can get very unsettled by antibiotic treatments and so rebalancing with good bacteria can make a difference. http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=21534 is a good choice because the paste also has kaolin and pectate which will firm stools and soothe the gut while the good bacteria are there too...
And agreed that a probiotic is a good idea, kittens' gut bacteria can get very unsettled by antibiotic treatments and so rebalancing with good bacteria can make a difference. http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=21534 is a good choice because the paste also has kaolin and pectate which will firm stools and soothe the gut while the good bacteria are there too...