There was Diluc in a distance observing you as you were talking to Kaeya. Kaeya was using his usual flirts and charms on you and here you were falling for them.

Diluc couldn’t help, but feel..jealous. Diluc has told a secret to Kaeya that only he knows: Diluc is in love with you. He wanted to confess his love, but seeing Kaeya flirting with you makes his stomach turn. Why was Kaeya doing this?? Especially in front of Diluc?? Diluc started to get angry, but also sad seeing you flirt back..