Ok Glow sisters! I need your help.

I am a person who gets my periods like clockwork.  Every 28 days. I was supposed to get my period on September 6th but nothing.  It is now the 8th and still nothing.  I took a pregnancy test yesterday and got a BFN.  I don't get where the heck my period is then! This has never ever happened to me.  I have one child already.  When I was prego with him I knew the day of my expected period. HELP!!