Period is late?

So my period is 3 days late. It's very regular, never had an issue but once before. Regular 28 days and have never been late a day, only early one or two before. I've been having small cramps here and there. No Spotting. I haven't had sex since January 2015, so pregnancy is out of the question. The only time I've ever been late was when I thought I was pregnant February last year and I stressed myself so bad I missed a week. Not stressed really now. When should I be worried enough to see a doctor?