Katsuki always found you going in the boy's changing room last making sure nobody saw you, which he found suspicious.. very, suspicious.

And so, he decided to stay and find out what you do in the changing room that you so anxiously always wanna be alone when doing.

He saw you enter and observed you looking around to see if anyone was there. 'Damn.. do they do drugs or some shit?' He thinks.

Then, his question gets answered when he sees you wearing a binder. “...what..”