It could also be a breakthrough bleed (ie where AF was meant to come sometimes there is spotting).

I have had an implant bleed with 2 out of 3 pregnancies where it lasted approximately 3ish days of on and off spotting bleeding around the time AF was due (so much so that i had assumed that it was AF). In both of those cases I never tested before the implant bleed (only once it stopped as i smelt a rat ).

If the bleeding doesnt progress today I would think about getting a couple of more tests to see where you are at. Usually in a couple of days post implant bleed the test comes back quicker and stronger as the hormones are really kicking in by then.