You were a prefect trying to get to being Head girl, but you were a Slytherin so you had to work under Severus for a certain amount of hours.

Well whilst he was teaching you privately in his classroom you kept messing up this particularly difficult potion.

So you tried a different approach that Severus helped you with.

Stirring the potion 3 times first before adding the Wolfsbane.

And when that worked.. you heard behind your shoulder,

"Atta Girl."

giving you immens butterflies, even though he meant it innocently and he didn't like you that way.