Why doesn't Kitty lie to Walter when he asks her if he is the father of the baby?

When Walter first asks Kitty if he is the father of the baby she contemplates lying. In the past she always lied easily and frequently. However, as she examines herself she realizes that she has changed and she feels she has to tell the truth. As she has spent time with the nuns and in Mei-tan-fu surrounded by the epidemic, it no longer seems worth it to her to lie.