You were in Narcissa Malfoy's room, as the two of you played fight. Narcissa was obviously stronger and bigger than you, but she made you believe you could beat her. At one point she pretended you had hit her too hard, and fell on her bed as she grabbed her shoulder and exclaimed "oww".

"Cissy, did I hurt you?".

Narcissa took advantage of the fact that you had leaned over her, and in one swift movement, grabbing you by the waist with one hand, she pulled you under her, using her free hand to keep your hands firmly clasped above your head.

"No dear, but maybe I will..." Narcissa said and kissed you violently, as her hand moved down to your center.