You were Oliver Wood's girlfriend and you have a animagus, it was a black cat and Oliver wanted to scare you so he came slowly in your dorm and found you in your black cat form, sitting on your bed.

And he approached slowly to scare you..

"BOO!"

He exclaim making you squeak and jump far away and Oliver laugh and you groan annoyed, still in your black cat form as you were now grumpy.