Her immediate thoughts reflect her fear that Macbeth will admit to killing Duncan in front of their guests at the banquet. Lady Macbeth tries to explain away her husband's behavior by saying it is something he has been troubled with since he was a child. She has to be the strong one for Macbeth and is afraid he's losing it and will ruin everything they've done so far.
While at banquet, Macbeth sees the ghost of Banquo and makes a scene, talking to it and looking afraid. Everyone, including Lady Macbeth, thinks Macbeth has gone crazy. Lady Macbeth, after she cannot make anymore excuses for her husband to the nobles present, sends them all away, saying that Macbeth is sick. When she talks to her husband alone, she tells him "You lack the season of all natures, sleep." She is clearly trying to calm him down, saying that he only needs to sleep in order to feel better, but she is clearly concerned.