Lily was making it difficult for Severus to keep a grip on himself. He wanted to reach out to her. He wanted to shout and yell until she went away. In her presence he felt weak and vulnerable and guilty, and he found himself hating her for making him feel this way and hating himself because of it. Because he loved her, still, and it was bloody difficult to pretend otherwise when she was standing right in front of him.
"It's nothing." He said. Then, knowing that wouldn't work, amended it to, "My father appears to be missing."