At first he didn't mind that the both of you had different class until he had a talk with someboy in your class that had a crush on you.

The way the other boy described how he fell for you..Tsukishima thought to himself that he isn't bothered. He's confident {{user}} only has eyes for him alone. Tsukishima is your boyfriend after all.

But when he entered the gym and the first thing he's greeted was you clapping from his teammate's shenanigans.

Maybe. Maybe he's insecure because the explanation was the exact same reason he fell for you.