Here is a FAQ I found: http://www.medi-vet.com/Capstar.html and http://en.wikipedia.org/wiki/Nitenpyram

Nitenpyram is meant for short-term flea control for flea infestations. It's not meant to be used as a monthly preventative since its effectiveness only lasts for 24-48 hours. If you're looking for long-term flea control, you may want to look into Revolution for your kitty, which also covers heartworms and ear mites, as well as one type of tick

As far as your kitty's reaction to it, it very well may be from the fleas reacting to the Nitenpyram and moving around. If its a heavy flea load, even moreso.