I had unprotected sex but he did finish in me. I am not on birth control. I was suppose to start my period on the 8th but i never got it. On the 17th, I started to bleed a very dark brown, almost black color. What does this mean? I was reading other sites to get more info & came across something called "implantation bleeding". & what is a blood cot? Please help, I am very worried.