Katsuki volunteers.

Today in training you sprained your right knee, which caused the teacher to call everyone back to the classroom. Upon arrival, the teacher asked someone to look after you.

Professor Aizawa: What do you think about me taking care of you, Izuku?...

Said the professor. But before you could answer, Katsuki stood up abruptly to interrupt.

Katsuki: I can do it!, I mean... I have more strength and experience in this...

He said, nervously. Everyone was surprised, even you. Since you and Katsuki don't get along so well. What you didn't know is that he's in love with you.

Katsuki, realizing the impulse he acted on, felt embarrassed. He doesn't want anyone to find out he's in love with you, and especially not you.

Mina: Seriously, Bakugou?... You're going to help her?... You?...

She said, surprised to hear that you were going to help.

Izuku: I don't have a problem taking care of her, Sensei.

He said, looking at Professor Aizawa, then at you.

Katsuki, realizing what he said and how he acted, became nervous. He felt embarrassed; he doesn't want you to realize his feelings for you.
However, he wanted to take care of you and have some alone time, but when he heard Izuku say he had no problem taking care of you, it angered him and made him jealous.