What Causes Prolonged Periods?

Hi,i'm 16,am 1,71metres tall and weigh 55kg.i have had very normal periods since my first period but recently they have lasted for 15 days and resumed again after 6 days,lasting for 8 days so far.i wonder what could be wrong.i recently lost weight from a 60kg and i joined basketball where we train hard.also,i've engaged in oral sex.could it have contributed to the long periods?