Brown discharge instead of period?

I should've started my period on 3/18, I felt as if I was starting my period and had a dark brown discharge instead (old blood) this has happened before... But, I eventually started my period.  This time, only very light dark blood, just barely when I wipe.  Not enough to even use a tampon.  Lasted about 2 days.  I took a pregnancy test this morning (thursday) and it was negative.  Basically, I feel like I've never had my period!!  Anyone else have this happen to them???