Because she really was disfigured in the previous life. Now it wasn't clear why Nie Li didn't recognize her chi. It's possible that it's because he didn't really know her that much in the previous life.
What happened was, some time after the first girl died, Nie Li met a masked Ning'er, who absolutely refused to removed her mask no matter what. Nie Li only realized it himself somewhat recently in the novel.
She's basically dreaming of a memory from her previous life.