What Causes Brown Vaginal Discharge While HPT Is Negative?

im 23 i have had irregular periods my whole life... but this period fell right on time but it was not like a normal period?? I had this go on for bout 4 or 5 days it started out light brown sometime dark i didnt have any major pains like i do when i usually have a period. took a pregnancy test it said negative.. i never had to use a tampon or a pad.. ive read up a little bit on the symptoms i had really only thing i found was this Implantation Bleeding so i dont know can you help?