Any idea whats wrong with me?

im always feeling tired and i have no motivation to do my studies at 6th form, my grades are slipping just as i need good grades to get into university in september, all i can be bothered to do is talk to my mates and go shopping or hang out with my friends. i have no energy to do any clubs or anything...i eat 3 normal meals a day but i snack like crazy in between, anything sugary normally, though today im craving cheese....i am happier than i have been in a long time (i have good friends, a good family and a fab boyfriend) but i just have no motivation to be academic....