A week ago I had unprotected sex using the withdrawl method, and immediately after I took Plan B. Today, exactly one week after the accident, when I went to the bathroom there was some very light pink/brownish blood when I went to wipe. However, nothing is showing up on a pad that I put on. Could this be implantation bleeding , or a side effect from the Plan B? It s still a week before my period is expected to be here.