I'm convinced that this is one of the things that happened the night of Bianca's party. If you remember, before that, Justine was with Thomas because she had a condition that made her basically insane, and Thomas feeding on her made her even out. Yet after Bianca's party she went for years without being fed on and she was just fine.
I've been convinced that Justine was be nemfected since nemesis was identified; it allows beings act against their nature, and Justine was suddenly this totally cool and put together person. I'm convinced that this is another one of the 'several things of significance' that happened that night, of which Harry was unaware. Her saying that it was when she started getting close to Lara was just another ploy, imho, to get Harry to think that Lara is the one who infected Justine (You know, since the ploy of getting Justine into the depths of Demon Reach was clearly failing).