What Causes A Delay In Menstruation?

I am 33 years old. I have always had regular cycles. I am a little concerned and trying to decide if I should wait it out, or contact my Gyno immediately. My last period was on April 23, and lasted the usual 7 days. I should have started my period on or around the 20th of May. I track my period and average between 27 and 28 day cycles. I have had my thyroid tested in the last 2 months and have been told it is fine. I don't feel that my stress level is anymore than usual. I have taken 2 pregancy test, both negative. The only time I have ever missed a period is when I was pregnant with my daughter (eight years ago). I am concerned that it could be something serious. Any ideas what could be going on? Should I wait a little longer before contact my doctor or go ahead and set up an appointment? Thanks for any info.