It seems that being queen isn't everything Lady Macbeth thought that it would be. She and Macbeth are now in power, as they had hoped for, but they seem to be drifting further and further apart from one another. Macbeth is making important decisions, like arranging for the murder of Banquo, without her input and even without her knowledge. In Act Three, scene two, she says,
Nought's had, all's spent,
Where our desire is got without content:
'Tis safer to be that which we destroy
Than by destruction dwell in doubtful joy. (3.2.6-9)
Here, she seems to say that they got what they wanted when they killed Duncan -- they are king and queen -- however, they are not happy, as they expected to be. She says that it might be better to be what they have destroyed that to be destroyed themselves and lacking happiness and contentment. It is pretty extreme that she now believes that it could be better to be dead than to be feeling the way she feels now; we get a bit of foreshadowing here.