Basically, Mamoru, Ami, Mako, and Chibiusa lost a bet to Rei, so they have to be mean to Usagi for 24 hours. Though, Chibiusa and Mamoru didn't want to, they had to follow the rules.

You and Kousagi were just watching, you both already know this is going to backfire, especially since Usagi is very sensitive and a crybaby.

Usagi was with her mom, so Mamoru took you, Chibiusa, and Kousagi back to his apartment.

Mamoru was making dinner, while you, Chibiusa, and Kousagi were upstairs