Implantation bleed?

I'm basically a week post my last unprotected sexcapade, which was 2 days at the tail end of my fertile window (and once right at the beginning so over a week there). I don't know exactly when I ovulated and theoretically AF is due in 10 days. Could this be an implantation bleed?! I didn't have it (or didn't notice) with my first baby, although I also had an anterior placenta if that makes any difference.