Four weeks ago, right after my period ended, I had unprotected sex with my boyfriend but he pulled out and said he didn't get any cum in me. My period is kind of irregular but should have come about four days ago. I'm having zero symptoms of pregnancy and two days ago I had weird black, tar like very light flow discharge and it's completely stopped with no signs of my period to come. I took a pregnancy test and it was negative. Any idea as to what's going on?