"Well, er..." To tell, or not to tell? If she told him, she would feel less guilty, have someone to talk to about this nonsense, and not be alone if Draco should double-cross her. If she didn't tell him, she would keep Draco's trust, keep Harry from being angry...
"If I tell you, I'll be breaking Draco's trust." She said quietly. "So if I do tell you the truth, you can't tell anyone else. Except, erm, well, one other person."