Implantation bleeding??

My period was due yesterday and on Friday I had light brown spotting which I assumed was my period. Usually my periods are heavy and accompanied with mood swings, cramps, bloating and abdominal pain. I have none of that. It's Sunday now and my "period" hasn't progressed further from the brownish discharge and over the course of yesterday was light enough to only need one liner. Is it possible this could be implantation bleeding even if I took a pg test on Friday that said negative? I really felt pregnant this month and I have NONE of my normal period symptoms. I'm so confused! Help!