8 days late but BFN? 😭😭

Okay so I had an HSG test last month, everything ended up being good. My doctor said we would wait until my next period to start letrozole and monitor my follicles before our IUI. She said if my period doesn't come by CD 30 to call to get a pregnancy test done. Thing is I usually have 21 day cycles, today is CD 29 and no period in sight. Ive been on metformin since august and have never been this late before. I've taken tons of pregnancy tests and they're all BFN's idk what's wrong with me? Is it even possible that I could be pregnant?  I bet this one glimmer of hope will be shattered and AF will probably show up immediately after setting up the pregnancy test at the doctors 😭😭😭