beatrix- i have heard several people say this about the impantation.
i personally am not on it as i have recently just had a baby. but before i was pregnant i was on depo provera which is the needle every 3months and i found i would sometimes bleed or spot after intercouse with my DP, or even sometimes just randomly without any sexual contact!
i was thinking about getting the implantation as i fell pregnant whilst on depo provera but i have heard so many stories on break through bleeding that i have opted not to.
maybe you should go see your dr and discuss it
goodluck