HELP!!! I need opinions!

I have a few questions.. Hopefully you guys can chime in and help! I have taken several home pregnancy tests, only one being positive. I just had a blood test done on Monday and it was negative. The Dr. I saw thinks my levels may have been too low to detect on a blood test? I am 16 days late, and I am never this late. Always on time. I am having brown discharge today, (not heavy), and I had brown spotting when I was supposed to be on my period. I have all classic signs of early pregnancy: extreme fatigue, swollen/tender breasts, extreme bloating, headaches, morning sickness and my cervix is really high. Has anybody had problems with hcg levels just being too low? My mom had that problem with me and my brother while pregnant, so I'm not sure if maybe I've inherited that or if it could be something more... Any thoughts?