What Causes Brown Vaginal Discharge While UPT Is Negative?

hi i am on birthcontrol and today is my second day of white reminder pills for my period for the past two days i have been cramping and giving off brown discharge andf cramps. i am on yaz andi took a pregnancy test last week and yesterday and both were negative but i took them around the time of my missed period yesterday was the first start of my period but still no blood but negative pregnancy tests and still cramps what could this mean? does yaz do this to you? do i count the discharge as my period?