First step would be to take a pregnancy test, just to see what it says. Best to use a pink dye test such as First Response, and use first morning urine. If that's positive, then go see your doctor for blood work. Many other things can delay a period though - stress, hormone imbalance, infection, illness, sudden change in weight or diet, late ovulation etc. So could just be one of those reasons. If the test is negative and in a week or so you still don't see your period, go see your doctor.