What Does Delayed Periods And Negative Pregnancy Test Suggestive Of?

Unprotected sex for a year and a half, my periods have been like clock work. I had my last period 2/19/15. I had sex a week after ovulation and I am 4 days late on my period (should ve started 3/20/15). No pregnancy symptoms, 3 negative urine tests and I m not sure why I m late.