Discuss the reasons behind Lady Macbeth's anxiety and sudden change of behavior after Act 3.

Up until Act 5, Lady Macbeth is firm in her belief that she and Macbeth are taking the correct steps towards achieving power. She frequently calms Macbeth's anxieties and pushes his ambition further, as highlighted in the following lines of Lady Macbeth's:

That which hath made them drunk hath made me bold. (II.ii.1)

These deeds must not be thought / After these ways; so, it will make us mad. (II.ii.45-46)

The sleeping and the dead...

Up until Act 5, Lady Macbeth is firm in her belief that she and Macbeth are taking the correct steps towards achieving power. She frequently calms Macbeth's anxieties and pushes his ambition further, as highlighted in the following lines of Lady Macbeth's:

That which hath made them drunk hath made me bold. (II.ii.1)

These deeds must not be thought / After these ways; so, it will make us mad. (II.ii.45-46)

The sleeping and the dead / Are but pictures. 'Tis the eye of childhood / That fears a painted devil. (II.ii.69-71)

Come on, gentle my lord, / Sleek over your rugged looks. Be bright and jovial / Among your guests tonight. (III.ii.30-32)

In many ways, Lady Macbeth may be more ambitious than Macbeth, who shows anxiety and regret over his actions early on in the play. Lady Macbeth is confident in their choices until after Act 3. Beginning in Act 5 - she does not appear in Act 4 - Lady Macbeth is immediately wracked with guilt. A doctor and gentlewoman discuss her symptoms ("I have seen her rise from her bed, throw her nightgown upon her; unlock her closet, take forth paper, fold it, write upon't, read it, afterwards seal it, and again return to bed; yet all this while in a most fast sleep," V.i.4-9). Lady Macbeth then sleepwalks onto the stage, confessing her crimes and attempting to scrub imaginary blood from her hands. 

There are a handful of justifiable reasons why Lady Macbeth has a sudden change of behavior in Act 5. The most obvious reason is that her guilt catches up with her. Blood is a symbol throughout Macbeth, often provoking fear and anxiety, and Lady Macbeth is preoccupied with imaginary blood in Act 5. Additionally, many of her earlier lines foreshadow her mental collapse. As highlighted above, she continuously tells Macbeth there is nothing to fear and that only children are afraid of the devil. Ultimately, Lady Macbeth is proven wrong and she emotionally unravels by the end of the play, resulting in her death.