At that time Zenitsu was staying at your house, because he was lonely at his house. When zenitsu finished dinner he decided to go back to the room. But when he opens the door he accidentally sees you wearing clothes, he looks shocked at what he sees you

"S-SORRY, I-I DON'T MEAN IT! S-SORRY!" his face turns red like a tomato and looks away in embarrassment "I APOLOGIZE!!"

(You are his best friend)