What Causes Blackish Vaginal Discharge After Having Unprotected Sex?

Me and my husband had unproceted sex on 8th day of my cycle and i noticed blackish discharge on 9th day of my cycle and 10th day too that is today itself. I had irregural periods .i wanted to know whhat does it mean and i had very painful periods everytime and trying to conceive from last three months.please suggest me i am very worried....