J_and_D could it be implantation bleeding? I'm sure (though it's been years) when I read up on IB it said you wouldn't get a positive hpt for a couple of days after IB. If you implanted late, say 11/12dpo it would account for blood working its way out at 12/13dpo and you may not get a bfp until 14/15 even 16 dpo. I don't like to build peoples hopes up unnecessarily, but it could be a possibility.