The main quest requires you to go to Koko's room every day for a week to talk to her. If you do, you get a good ending with her. If you skip one or more days, you get a worse ending.
If you've been going every day and you've gotten the good ending where she kisses you, you've done everything right, she must just have gotten mad at you for skipping the tea ceremony on Sundays.
This is a mistake on my part, I guess I didn't cause the club events to be cancelled for that Sunday.
Don't worry about it, it doesn't affect your relationship with her. Next Sunday, if you go to the tea ceremony, she should be back on good terms with you.