Ttc- late but bfn

Tomorrow I will be 8 days late. I have been testing every other day or so and getting bfn! I just turned 40 in January. Increased appetite, slight weight gain this week, nausea, headaches, frequent urination (I always pee a lot but way more now), constipation and fatigue. So, why am I not getting a bfp? I have a 16 year old from a previous marriage and I had a miscarriage 20 years ago. I'm not sure what to think.... Anyone??