Of course she thinks he's lying on the rare occasion that he tells the truth. A partial one, but still more than he was planning for fear that she may not treat him the same if she knew. And if she needs someone to be angry with, he understands. It's better him than someone who didn't take it so expertly well.

Spike walks to her hand, like she asks, but doesn't respond otherwise as he pays close attention to her struggling. He's guilty of what she says, and there's nothing that he can claim to deny that.