What does Buffy do upstairs?

She gets overcome by something

As she climbed the stairs of her house, Buffy felt guilty. She should have told Willow the truth. But she had a hard time believing that even Willow would believe her.

She went into the bathroom and starts to undress, first by taking off her jacket, then by unbuttoning her blouse. Next she unzipped her jeans. As she stripped down to her underwear, she noticed the purple bruises on her skin that contrasted quite strongly with her white underwear. Especially prominent was the bruise on her forehead. And, of course, the bite mark on her left shoulder.

That's definitely going to leave a mark, thought Buffy as she unhooked her bra.

She took off her bra and started to take off her underpants. Suddenly a wave of nausea swept through her. She clutched her stomach and collapsed upon the ground.

"Help!" she cried, but somehow she knew that Willow would not be able to climb the stairs fast enough to prevent what was happening. She could never be fast enough. Not even if she possessed the strength and speed of a slayer, which she didn't.

She heard the sound of ripping fabric as her body changed. Then she felt the touch of the cold bathroom floor upon parts on her body that had been modestly covered just a few seconds ago.

"What is it?" she heard Willow shouting.

Buffy tried to shout back but whatever it was had taken over and she could not speak. There was a buzzing in her head -- a loud buzzing -- like bees from hell. No, killer bees from hell. No, giant killer bees from he-- Gosh darn it, she had been listening to Xander too much.

Then a wave of darkness swept over her and she lost consciousness just as she heard the approaching footsteps of her best friend.