It could well be, but bare in mind, implantation bleeding isn't very common. Other things can cause this to happen - infection etc. All you can really do though, is wait till the 21st and if your period doesn't show - take a pregnancy test with first morning urine. You can test a few days before your period is due but you're best not doing so - it's so early to test and you might get a false answer. Good Luck.
Hey, I am suppose to start my period on the 25 and I noticed what I thought was implantation bleeding last friday, and now today I am leaking milk....idk what is goin on but I think maybe you could have implantation bleeding =)