Confused!

My cycle is pretty accurate and really regular, every 28th day I start. I'm not one to ever be late. My husband and I have been married for 5 years and never once used protection or any other forms of birth control. The last few months I have been using and updating this app with my periods. I was supposed to start last Friday, the 13th. And here I am getting negative results! Yesterday and Monday I had a very few light cramps, but no spotting at all. What could be going on? Any advice please!