1st...read the instructions. It must be for cats (you know that) but check for an age on the label, some vary.
2nd..fleas don't LIVE on the pet, they live in the environment. They FEED on the pet. So, if it is freezing outside, then they must be alive inside.
3rd...Fall is your prime flea season. This is when the weather cools enough for the fleas to seek a warm place to spend the winter.

If it is only a couple, don't panic. Get them off the pet. Treat any area in the home that you suspect. Even a good vacuuming with disposal of the bag immediatly outside, will help. And keep an eye on the pet because fleas are very persistant and can be hard to get rid of. If just one pregnant female survives the cycle can start all over.

Good luck