4 days late...

Megan

So My period is 4 days late and I have always been pretty regular. My period is usually the same time every month sometimes I might start a day late never 4! I have taken 3 tests one on Friday, one on Sunday, and one today and they have been negative. I was suppose to ovulate the beginning of this month but usually I can feel when I ovulate and I am pretty sure I did the following week and had sex on the 10th and 16th. Any ideas on what might be going on and stress level is the same as usual.....