Dark spotting/bleeding

Michelle • Mother of 1 • Engaged to my high school sweetheart • TTC #2 • Georgia born and raised•
Just curious... 
My period was 15-21st this month. It was normal... And now for the past 3 days I've been having dark bleeding/spotting. I think it's stopped but what could that be? Ovulation? Or what?
It started exactly 7 days after my period. But glow says my fertile window isn't until 09-15th of JULY .... ???