How Is Menstrual Bleeding Different From Implantation Bleeding?

I had my period December 13th and had sex on the 20th of December (when I was ovulating) it is now jan 6th 17 days after ovulation and sex and my period is due for jan 8th (so I haven't missed my period yet) I have light blood streaks in my pad and I cannot tell of its implantation blood or pre period blood.. I read that stress can cause a light period before the actual thing starts, and I've been stressed out about being pregnant. Can sperm live inside me that long? Do I have my dates wrong? Is this period blood or implantation blood? What do you guys think?