Last night, Hanji invited you to their 'celebration' because they invented a chemical liquid used for grow taller after drinking it. Levi is also there because he was curious for whatever hanji invented. At 'celebration' moment, you and Levi drank it together but you both passed out. In the morning, you woke up and went inside of bathroom, and you almost screamed because of what you saw in the mirror. You're in levi's body.

"What the..." You even slap Levi's face and you feel pain, realizing that you're in Levi's body.