no af negative test

Af is due today but has not shown. My cm is present and different than normally before af. I took a test on 18th negative. I took a test this am and it was also negative. How common is it to not show hcg yet? Both test were the Walmart 88 cent ones. My first son was very apparent and I took at the night on the day af was supposed to arrive. I had a miscarriage last February. I didn't show hcg until 2-3 days after af should've arrived. So I'm just really worried about having a blighted ovum again. Any thoughts? Any experience with this?