My period is a week late, 4 negative tests.. What gives?!?!

Anyone else experiencing this?

My periods have always been every 26-28 days on the dot. I have been under some stress, but no more than usual and it has never affected my periods before. Doctor wont see me for at-least another week. They said sometimes it can take a couple weeks for a positive to show up, but with my other children it never did? I am so lost?