Ob-Gyn questions? Ask a gynecologist online.
Implantation bleeding seems highly unlikely, mostly because pregnancy is highly unlikely having sex while on your period.
Most of the time when women think they may be having implantation bleeding, it's just irregular spotting.
If your period comes late, I'd do a pregnancy test, but I doubt that you're pregnant.
If your period comes when expected, you'll know you're not. Best regards.