Implantation bleeding? BFN..

Implantation or no? I had this yesterday when I went to the bathroom, and nothing since. I've had unprotected sex several times this month. I never start my period early, or spot. I am not due for my period for 3 days. (4 days when this happened) I took a early response pregnancy test this afternoon and it was a BFN. (I hate the wait, I'm anxious!) Do you think this is implantation? How long should I wait to take another to get accurate results if it is implantation?