This specific line refers back to the night of King Duncan's murder, specifically the moment when a knock at the door forced the couple to hurry to their chamber to avoid suspicion. Draft of the Full Speech Excerpt
This line highlights Lady Macbeth's transition from a cold, calculating figure to one "unhinged" by trauma and guilt. come_give_me_your_hand
For your reference, here is the immediate draft of that section from Shakespeare's Macbeth : This specific line refers back to the night