What Causes Delayed Periods With Abdominal Pain?

I'm 27 and I got married in Feb 2011. After marriage, i got my first periods as per my regular cycle of 27 days. But this month its delayed by a day and i feel some pain in my abdomen for the past 3-4 days. I feel like I 've got my periods but actually i dint. Neither me nor my husband are on birth control. Please advice.