Why do i feel so depressed?

on the first weekend of school i went to six flags with my cousins and it was really fun and the weekend of Friday September 18 was fun because i went to an engagement party far away and now im really sad its over. I had bronchitis during the party and i still do and its like i actually miss having that fever during the party its just stuck in my head... and thats all i think about and feel sad thats it gone.