My cat only had one kitten and this is her first litter.

The first litter is never a very big litter.Just like as she ages, she won't have big litters anymore..

You should take her to the vet to have an ultrasound done just to be safe.

As long as she is healthy and the kitten is healthy, you should have nothing to worry about. I guess someone's cat shoots off a lot of blanks ;)

You should get your cat spayed anyway. Lord knows this world is over populated by cats.

Having one kitten in a litter isn't unheard of, especially if it's a large kitten and/or a small mother. It's also possible that more kittens will be born later.

Can you feel any other kittens when you feel her stomach? If you're worried that there may be more kittens, or she seems distressed, then you should take her and her baby to the vet to be looked at.