Ringworm is not actually a visible worm you would see (it's a fungus similar to athlete's foot). If it's a tapeworm, your cat will need to be dewormed with something like "Droncit" or "Drontal." More importantly, tapeworm is transmitted by FLEAS, so it's important that your cat (especially if he's outdoor) that he be on a flea preventative as mentioned above (like Advantage, Frontline, etc.).
As someone who works in a toxicology field, make sure you don't use ADVANTIX on cats (it sounds similar to Advantage) - this is only for dogs and can be toxic to cats.
I also suggest looking for any wounds or stool encrusted areas anywhere as occasionally maggots may infest such spots!
If the worm was creamy white in the back, with a dark head, it may be a small fly larva (maggot) or a flea larva. Please make sure your cat is on a good monthly flea preventative such as Advantage or Frontline, and make sure she is not rummaging through decaying trash outside.
Kimberly Coyner, DVM