the possible causes for your bleeding are,
-side effect of implanon:
as sometimes this can cause menstrual irregularities, that may be reason for your heavy bleeding,
as there is around 1% failure rate for implanon, there is chance of pregnancy,
then your bleeding could be implantation
bleeding but normally that may not be this much severe,
so this bleeding can also be because of miscarriage
you better visit your gynecologist and get examined to rule out above possibilities.