Was this flea med for a dog that your cat got in to?
The chemicals in canine flea treatments (drops, shampoos, sprays, pills, and so on) are often Imidacloprid and Permethrin. Permethrin is toxic to cats and in the concentrations that are in canine flea drop treatments (or even pills) can be lethal.
The symptoms of poisoning are deceptive in that the animal may initially display drooling, vomiting, diarrhea, loss of balance, lethargy and other very obvious signs; however, they may then seem to recover and owners think everything is fine. What may be happening is that the toxin is causing liver damage, enough to be leading to liver failure, which can take a few days or even a few weeks to become apparent. By the time liver failure is diagnosed, it's often too late. Sometimes it's never diagnosed and owners end up finding their precious companion lifeless, not knowing what happened.
Find out more about liver failure here http://www.marvistavet.com/html/body_hepatic_lipidosis.html
It's strongly suggested that the animal be taken to a vet or emergency clinic immediately. Waiting for signs of poisoning may cost precious time that could be used preserving the cat's life. Early intervention is often successful. Waiting too long is usually tragic.
Bring the medication container with you if at all possible
Good luck and God bless,