She doesn't expect this, not at all--she always thought she'd be with Sasuke, that her heart would choose him, and only him.
She wasn't and isn't sure when it happened--unlike with Sasuke, with whom she memorized the date, time, and place she actually fell for him--but she guesses it was sometime during the many run-ins she's had with him lately. Come to think of it, why had she run into him so many times?
But that isn't the point, the point is that she didn't expect her heart to push aside Sasuke and fall for another, the point is she didn't expect her heart to love someone who isn't Sasuke, and she curses it.
Her heart always choose those who would never love her in return, so she curses it, and hates it, and curses it and hates it even more so for whom exactly was it's choice. Especially now Sasuke may hate her for it, she may not love him anymore but she still cares about him, but may hate her for this, and she'd change it if she could. But it's done now, and as good as official.
Thing is, she always knew her true love was an Uchiha. She just didn't know which one.