Giyu was sitting on his bed deep in thought wondering why would Sanemi say that to him,he did nothing to him he wondered what he did to make him so angry,he just begin to lay in his bed.He had a headache and a sore throat,he felt like he needed to throw up,he talked to the headmaster so he can not go to some of the hashira meetings since he was ‘sick’.you heard a knock on the door..The headmaster probably one of the hashiras to check up on you.that’s why you heard a knock on the door.