Just curious

My average cycle is 28 days lately 
I spot lightly probably 2 days or so before AF is due and then I'm full on for usually 7 days 
My period is due in 8 days (according to Glow a and another additional app I have)
My husband and I had I unprotected sex Sunday night 
I had very light spotting this afternoon and a dark discharge similar to the last day of a period 
What in the world? Lol
Would that be implantation bleeding?
That early?
According to glow I was outside of my fertility window that I know of
Never had anything show up this early before...