How does the point of view in "Lamb to the Slaughter" change?

Over the course of Roald Dahl's "Lamb to the Slaughter," the point of view is always third person limited. We as readers are told about events from Mary's point of reference for the entirety of the story. She is spoken about in third person, but we learn about her emotions as though she is the one telling us how she feels.
As the story progresses, though, Mary's point of view does change. Initially, she is in disbelief over Patrick's announcement, but after killing him behaves very calmly as she goes about deciding her next steps. She wisely decides to go to the store and talk to the grocer about her plans for making Patrick dinner that night, which will give her an alibi for why she could not have killed her husband.
Upon returning home, though, Mary suddenly behaves as though she really is discovering Patrick is dead for the first time. She becomes very upset and cries. This is likely because Mary is just now fully realizing that she killed her husband. In that way, it could be argued that Mary's own perspective changes throughout the story.