i am 13 years old and i noticed today that i have a dark brown (almost black) discharge in my undies. i have not started my period yet. for about 3 days i have had this terrible stomachache off and on. i asked my mom if it was because i was getting ready to start my period and she said that it might be. when i pee, it is light yellow but when i wipe it is light brown. what does this mean? am i starting my period of is it something serious?