Question about menstrual cycles?

okay, im a 21 y.o african american female.. i have an app on my phone that helps me determine my period and for a year this app has never been wrong, so im gonna say that i was 5 days late for my menstrual and then after that i came on, for five days with my period being darker(in color) and lighter( cause i usually have a heavy flow).. but i was on for my five days.. four days after my period i began spotting, has this ever happened to anyone.? what can it mean? i am not on any form of birth control.. please help