Is It Implantation Bleeding?

Hi.

I started my period on the 8th and had sex with my boyfriend on the 11th.
He assured me that he hadn't ejactulated (or pre-come) because it was so short. [we wanted to try it without a condom for literally a few seconds]
I've finished my period and on the 17th I noticed some blood. I noticed it some again on the 20th.
It was its normal colour but there wasn't a lot of it.

Could it be implantation bleeding?