Why was my cat dragging her butt on the floor?

Worms are actually very rarely the cause of this - and tapeworm is the most common worm to cause any anal irritation, but even that rarely causes butt scooting (but if kitty has no fleas and doesn't eat prey, kitty more than likely doesn't have tapeworm).

http://www.billingsanimalfamilyhospital.com/faq/

And as the site mentions, the most common cause of butt scooting and immediate licking is impacted anal sacs. http://cats.about.com/cs/healthissues/a/analglands...

But this only happened once and hasn't happened since?

Another possibility is that her poop was too hard to come out and caused irritation (though, this would usually cause bright red blood, from irritation in the rectum, but not always).

Or that part of it was stuck in her fur - they'll often 'wipe' their butt on the ground if it's stuck on them.

If she doesn't seem to be having any difficulties pooping or showing any of this behaviour since then, I'd brush it off as a hard poop until morning and call the vet and ask.

Majority of the time when animals drag their butts on the floor its because they have an itch,they may have worms, or they may be trying to mark their territory. Call your vet just to make sure.

She might be itching her butt, of poop was left behind then she might have poop around her butt. Wipe her butt with a cloth she may be itchy.