What's the deal?

Kristie
Hey everyone! 
My husband and I are TTC. I am 4 days late for my period which is very odd for me because my periods are very regular and come like clock work. I've taken a few pregnancy tests and all have come back negative. I have no symptoms of pregnancy or PMS-- I always get severe PMS symptoms and know when my period is on its way/arrived, but not this time. What's the deal?