Why does Lady Macbeth's character change?

It seems that Lady Macbeth's character undergoes a change when her guilty conscience becomes too much for her to bear.  In Act 5, Scene 1, the sleepwalking scene, she is clearly reliving the night of Duncan's murder, except now she imagines that her hands are still stained with his blood.  At the time, she'd said, "a little water clears us of this deed" (2.2.86); now, however, she says that "All the perfumes of Arabia will...

It seems that Lady Macbeth's character undergoes a change when her guilty conscience becomes too much for her to bear.  In Act 5, Scene 1, the sleepwalking scene, she is clearly reliving the night of Duncan's murder, except now she imagines that her hands are still stained with his blood.  At the time, she'd said, "a little water clears us of this deed" (2.2.86); now, however, she says that "All the perfumes of Arabia will not sweeten this little hand" (5.1.52-55). 

Further, it was Macbeth who initially feared that he would not be able to sleep anymore because he murdered Duncan while he was sleeping; now, it is Lady Macbeth who cannot sleep due to guilt. She remembers chastising Macbeth for what she perceived as weakness and cowardice, and she repeats many of these phrases, but they are peppered with references to Duncan's blood, Macduff's family, and her inability to wash the blood from her hands. In Macbeth, she's created a monster who will do anything to hold on to the power that he is taken by force, including things she never planned on, like when he orders the murders of Macduff's wife and small children.  While she sleepwalks, she says, "The Thane of Fife had a wife. Where is she now?" (5.1.44-45).  It seems that she she bears some of the guilt for these deaths as well, because it was she who coerced Macbeth to commit the first murder.

Also notable is the fact that Lady Macbeth no longer speaks in verse, as she has always done in the past.  Shakespeare typically reserves verse for nobility, but he also sometimes has characters speak in prose to indicate some kind of mental break. (For example, Ophelia, in Hamlet, speaks in prose after she's gone mad, and Hamlet speaks in prose when he wants others to believe that he's gone mad.) Now, Lady Macbeth speaks in prose, and this gives us some clue as to just how guilty she feels, so guilty that it has driven her insane.