In act 5, scene 1, Lady Macbeth is behaving as though she has a terribly guilty conscience, heavily burdened by the murder of Duncan as well as all the other terrible things Macbeth has done since this first crime. Recall that Macbeth had wanted to back out of their plan to murder the king but that it was Lady Macbeth who talked him into it again by insulting his manhood and pride, telling him he'd be a coward if he did not pursue the crown.
In her somnambulatory state, Lady Macbeth now imagines that there is still blood on her hands from Duncan's murder. She seems to relive the night of that first murder, counting the strokes of the clock the couple heard then as they awaited their opportunity to strike: "One. Two. Why then, 'tis time to do 't" (5.1.37–38).
She also has some awareness of what happened to the innocent wife and children of Macduff, as she laments, "The Thane of Fife had a wife. Where is she now?" (5.1.44–45). She realizes, on some level, that Macbeth is responsible for the Macduffs' deaths, so perhaps she realizes, too, the hand she has had in initiating their sad fate.
In her guilt, Lady Macbeth now cries that "All the perfumes of Arabia will not sweeten this little hand" (5.1.53–55). It is ironic, of course, that she once told Macbeth "a little water clears us of this deed" when he lamented the king's blood on his own hands immediately following the murder (2.2.86). She also chastised him then, saying, "Your constancy / Hath left you unattended," and now she is the one being inconstant and acting guiltily, even without her knowing it (2.2.87–88). She berated him for hallucinating, and now it is she who hallucinates.
The Macbeths have, evidently, created such a culture of fear with their reign that the gentlewoman who serves Lady Macbeth refuses to repeat what she has heard her mistress say, because she has "no witness to confirm [her] speech" (5.1.19–20). Frightened for her own safety, it seems, she will not even report to a doctor what the queen has been doing and saying in her sleep; the waiting woman will only bring him to see and hear for himself.