help explain why ?

okay, so i know im not pregnant.
i do marching band, like its hardcore.. 8am-9pm with a one hour break at 12:30 and a two hour one at 4:30. im barely eating bc i dont have time to go home so i eat small food like hot pockets, oranges, things that can be digested and im drinking soooo much water. i got my period yesterday?? and its lighter than usually, can someone help me know why??