When Macbeth starts to become increasingly agitated and troubled by the guilt of his actions, he questions Lady Macbeth's behavior, saying she seems "unwell" and "sick at heart." Lady Macbeth quickly deflects his concerns by saying she was simply "too full of the direst cruelty" to properly process the events and that she "did unseam them" (meaning she ripped open their insides) in her "thoughts." She then tells Macbeth she "will have these things that trouble me / Be gone tonight" and claims she was just suffering from the heat:
> "My hands are of your color, but I shame / To wear a heart so white. [Aside] I hear you knocking at the south entry. — / Retire we to our chamber: a little while, — / I pray you."
She then excuses herself and leaves to compose herself and attempt to hide her true feelings.
This excuse is important because it demonstrates how Lady Macbeth is trying to maintain control and project an image of strength and composure in the face of her own growing guilt and fear. She is not truly sick, but rather, she is trying to mask the true nature of her internal struggle with a more acceptable explanation.