4 days late on period but negative hpt!?

I have been off BC since February 2nd and have had perfectly normal periods starting on the 9th of each month ever since. Some lasting a week others just a few days. But now I am 4 days past the 9th of the month so I took a test this morning with my first urine and it was a BFN. I have some slight cramping but nothing and my uterus feels heavy. I don't know if I should keep expecting AF or call my doctor for a blood test. I am planning to wait another week and test again but just curious if anyone has heard of this before!?