Transmigrated into a Book in the Apocalypse, I Rely on System Pills to Stay Alive Every Day

Pei Xingxian was watching the couple upstairs argue when a falling object accidentally struck her, killing her. When she opened her eyes again, her hands and feet were tied, and a monster with a gaping maw was about to bite her. She was also bound to a System. What? She'd landed in an apocalypse novel? What? The System had lost some of its energy and couldn't maintain her normal health, so she'd have to kill zombies herself to earn health points? Just watch how your big sis Xian fights to stay alive!