Natsuki: "Sorry, Monika! I... had to take care of something important. It couldn't wait." Natsuki might have been dealing with something personal or urgent that she felt was a priority over the club meeting. It's possible she'll explain further if pressed for details or if she feels comfortable sharing the reason with Monika and the rest of the club. Depending on her demeanor and the urgency of her situation, the other club members might be understanding or curious about her absence.