implantation bleeding does not occur a couple weeks before you miss your period, and neither does it occur once you are pregnant.
you typically ovulate about 14 days before your period is due.
this is when the sperm can fertilise the egg.
the fertilised egg then continues it's journey down the fallopian tubes.
this typically takes about 10 days.
so for an average person with a typical cycle implantation bleeding (which si not common and definately not experienced by everyone) happens around 4 days before your missed period.