I had this with my second, we even did 2 blood tests. They finally confirmed at 11 weeks via ultrasound. Never had a positive test. That's so rare, but some of us just have low hcg.
I had this happen to me in August. I was 5 days late and kept getting BFN (3 actually). I am usually pretty regular, but it turns out that I had just ovulated later than I normally do so my period came later too. I got my period on the 6th day. But you can always get a blood test from your doctor to confirm. From what I read online when it happened to me it is likely that you ovulated later, but there is also the rare occasion that the tests just aren't catching your low levels of hcg like @obimoma
Also, many people suggested that I start charting my temp so that I would know when I ovulated in the future.
Re: 5 days late