This happened to me just this past month. I have been regular as long as I've been getting my period, even after 2 kids. At 4 days late, I took a negative PG test. Called the doctor, was told wait a week, take another test, reason being, it could be a fluke and I ovulated late, meaning I'm either not pregnant or my hcg levels weren't high enough yet. At 11 days late, took another test, negative. Doctor told me to wait another week and we'd then do a blood test. At 13 days late I got my period. So who knows? I would call your doctor, though. If it's driving you crazy, just ask for the blood test.