Upon receiving pain, her animal instincts had taken hold and caused a 'partial shift'. It was a logical reaction when you took into consideration the nature of the animal.
"But never of your own volition?" Hannibal asked, although he already knew the answer. He paused for a moment before continuing "If I were to ask you to shift now, would you be able to?" he asked. It would show some element of control if she could. If she could but refused to it would be her fear that was taking away such control.