What Causes A Brown Discharge Post An Unprotected Sex?

Me and my partner only have had one partner which is each other. we had unprotected sex a few days after my last period ended. last week (almost a week before my period) i had dark brown discharge for three days. right now i'm not having any pms symptoms and i start my period soon. but my vagina is kinda tender and swollen and earlier i had pressure on my ovaries. what could this be?