She can't cope with it; it's utterly unlike Clark not to be concerned with her pleasure, as she is with his own in return. Her body stiffens again, perhaps not noticeable at first, but the tense way she holds herself under him might eventually penetrate whatever is going on in his head.

There is an audible signal as well, in the form of her first sob, and then words. "Clark .. what .. why are you--"