The day Tay met Georgia Mason, she was infected with the Kellis-Amberlee virus. And, unbeknownst to both of them at the time, the virus decided that the shiny new playground of Tay's new physiology was the perfect place to go live, and Tay began going spontaneous amplification. In Tay's favor, however, was her angelic immune system and the fact that the virus isn't quite the same as it was before passing through the rift. So, instead of locking herself in her room and turning into a zombie, Tay locked herself in her room, started feeling sick, and promptly slept for about three days straight.
She wakes up with her head under the blankets and feeling pretty much good as new, if ravenously hungry and kind of needing to pee. She pulls the blanket down, opens her eyes--
And promptly lets out a startled, pained shout when the light streaming through her window sends stabbing pain shooting through her eyes, prompting her to squeeze them shut and pull the blanket back over her head. After a few minutes of cursing and pretending she's not practically whimpering because that FUCKING HURT, she gropes around for her journal, opens her eyes a crack long enough to find a blank page, and
scrawls an entry with her eyes closed.
It's not until after she writes it that she realises Xander and his "dark things" won't be able to get in, and she shuffles to the door long enough to unlock it before burying herself under her blankets and preparing to wait for Xander (or someone) to come to her rescue.