...all he did was to follow the tradition and hold an Unbirthday Party at Heartslabyul dorm.

Somebody had mixed the tea with whisky on purpose (what a cheap prank) and so Riddle, who doesn't even know how alcohol tastes like, drunk his cup in a sip. After the third cup of "tea"...something started feeling off.

Riddle starts acting childish, stubborn, grumpy and coy in a funny way...

Yup...He is completely drunk.