What Causes Delay In Periods?

I took an ovulation test in Dec 2010 and it showed the smiley face on day 14 from my last menstrual cycle. I usually have my cycle 28 to 30 days today is day 31 i have taken 3 pregnancy test and all 3 have been negative any suggestions....I go on Friday to see a doctor for the first time I am 38 years old and never had children my husand had a vasectomy 11 years ago after his 2nd child from his first marriage he had it reversed on Nov 4 2010. I am just hunting for some answers