Spotting three days but no period.

I normally have very regular periods starting on the 9th of every month with heavy flow, so this month I was very surprised when I had only very light spotting on day I was supposed to start.It was kind of dark brown and not even enough to see on toilet paper most times when I wiped and it only lasted three days.Anyways, I figured my AF was just a couple days late so I waited and now it is the night of the 15th and still nothing.Any ideas of what this could be.I have no cramping or anything like that and it was weird leading up to when I should have started I had no backache, headache, or cramps as I usually do premenstral,I am not on birthcontrol to miss any to mess up my cycle and I have had the same sleep level,physicality level and normal amounts of stress that I usually have, in saying that I mean to say that nothing about me or my environment has changed.So I have no idea what this could be.Please help.