Quite likely a combination of guilt, worry, and lack of sleep (with nightmares). He doesn't sleep well immediately after killing King Duncan, and he says his dreams say, "Macbeth doth murder sleep", which can't be reassuring! Both he and Lady Macbeth do not sleep, and she sleepwalks. He would worry about her, in the sense that he loved her, but more so about what information she might impart while sleepwalking. He feels guilt about Duncan, and more after Banquo's murder. He worries about Fleance's escape and the fulfillment of the witches' prophecy that Banquo's heirs will take his (Macbeth's) crown. He feels less guilt initially about the killing of Macduff's family, but when he meets Macduff in the final battle, his guilt shows up when he says he does not want to kill Macduff as well.
I would say his shift in character started after he became King. He was a little worried about being overthrown so he went to the Three Witches to ask what his future would be. The apparitions that came to him made him very confident and he got cocky and careless. He turned into a tyrant in which at the beginning of the play, the "old Macbeth" would have tried to take down.