Perhaps if you have just had a bit of spotting you O'd really late and are having an implantation bleed? I am suggesting this as last month (when I got my BFP - unfortunately it didn't last) I had lots of CM and did a test a little after I thought AF was due but got a BFN. About a week later I had some spotting and then tested about 4 days after that and got a BFP.....

Good luck!