It is very important to be fully aware of what has just happened in this important scene. Macbeth has just confessed that he killed the two henchman. This of course is a very dangerous thing to have done, as they could have been questioned regarding their "involvement" in Duncan's murder. However, Macbeth has silenced them for good, obviously to conceal his own guilt and to safely enable him to place the blame on them. This causes Maduff to ask suspiciously "Wherefore did you so?" In response, Macbeth gives a somewhat unconvincing explanation, citing his love of his liege and his automatic anger at seeing the "murderers" who were "unmannerly breech'd with gore." Macbeth is obviously flailing around for an excuse here, and Lady Macbeth clearly feels that he is not doing a very good job and so chooses to create a distraction by fainting, drawing attention away from Macbeth and his deed.
Therefore, most definitely this is a feigned, calculated move on the part of Lady Macbeth. She is seeking to help out her husband by distracting the suspicions of Macduff.