I know this is an old post, but I wanted to reply because there may be other women out there that have googled "spotting before perion after Plan B" as I did. I NEVER spot before my period so I was convinced it had to be implantation bleeding when I started spotting scant amounts of light brown discharge 3 days before my period. I took Plan B after a condom broke during sex on my peak ovulation day. Pregnancy test was negative, but I thought it was too early.
My period came on time, exactly 28 days after the last one. I guess the spotting was either due to Plan B, or it was something new that I can expect to happen now.
Hope this provides some comfort to anyone reading this who is worried that spotting may be implantation bleeding.
it tells you the effects with the instructions. bleeding is one of them