The Cautionary Tale Of Numero Cinco - S5-E6
Revealing mistake: When Angel says to Wesley that "Fred has got something," and then walks out of the room, Angel's reflection is visible in a window pane in front of him. It's very brief, but it's there. (00:31:05)
Answer: Due to his epiphany, Angel basically had the 'true darkness' he worked so hard to reattain ripped away from him, like his soul was after sleeping with Buffy.