What Causes Brownish Black Vaginal Bleeding During Menstruation?

Hello. I have been trying to conceive for the past 7 months following a miscarriage at 10 weeks when the baby's heart stopped beating. This month I had cramps on and off from ovulation to my period, which I have never experienced before. My period started 14 days following ovulation but for the first two days I expelled a dark brown/black clotty tar like blood. On day three I commenced a normal bleed. I'm just wondering what if I should be worried and what the dark tar like bleed could be from.