not too sure but the only time I have gone this long & still got bfn is when I have had an early m/c.  I have had a couple of early ones, one I tested early & got + then when went to dr it was really faint then af started a few days later, bit of a bummer as it was our 1st + since ttc so we had told our families straight away (we were living in Aust at the time) only to have to ring them a week later and tell them it was a false start then they had to tell everyone that they had told. 

Hope this is not the case for you and maybe the test is just playing a trick on you, good luck!!!