How does Buffy end up naked?

She gets bit by a vicious were-something

"My God, Buffy," said Willow. "Whatever happened to you?"

"I got attacked while I was on patrol," said the bruised and battered vampire slayer. "One moment I was walking through the woods near the old cemetery, the next I was being attacked by something. I'd like to think I gave as good as I got but I'm still surprised how powerful it was."

"So what was it?" asked Willow. "A vampire?"

"No, it was -- Actually I'm not sure what it was. Just that it was big and hairy and strong. I never fought anything like it in all my life."

"Could it have been a werewolf?" said Willow. "Like Oz?" Despite herself, there was a ray of hope in her voice.

"No, it wasn't a werewolf."

"Maybe you should call Giles."

"I will, but not right now," said Buffy. "Right now I have to go upstairs and undress."

"What?" said Willow.

"Calm down, Willow. I meant undress for my shower. I really need to take a hot shower after everything I've been through tonight. I'm sure you understand."

"If you say so," said Willow.

Yet as she watched her best friend head upstairs, Willow got the uncanny feeling that Buffy was hiding something.