Once Duncan's murdered body is discovered in the morning, both Macbeths try to be as natural as possible given the circumstances. Lady Macbeth fares better on the whole: she acts shocked ("What, in our house?") and even pretends to faint.
Unfortunately for their plan, Macbeth is losing his poise. Macbeth, in a fit of nerves, kills the guards he and Lady Macbeth intended to frame for the murder so they could not share any information that would discredit that story. However, his actions, though he explains them as the product of great love for Duncan, only cast suspicion upon him. The deed seems too rash for Macbeth given his public persona. This, added to his odd, distant behavior before the discovery of the corpse (if one examines his reactions to Macduff's small talk, he is clearly distracted and nervous), would make Macbeth seem suspicious right away. He comes close to incriminating himself through his strange behavior alone.
Lady Macbeth likely notices this and directs attention away from Macbeth by pretending to swoon. Her reaction is not as suspicious since she is pretending to be what all of the men assume she already is: a gentle, sensitive woman who cannot handle news of an act as vile as assassination. Unlike Macbeth, Lady Macbeth has a better handle on her audience, so to speak, and is a deft improviser when her partner-in-crime slips up.