what happen to me ? hi i born 8 may 1998 now i am 15 years old

now problem is that when i was 7,8,9,and 10 years old i was so intelligent and my Favorited game was soccer and i was best player but now i cannot understand anything in some subjects and i become so bad in playing soccer every year come it s going low and low in everything so what happen is something about my signs or it just simple happen life