If you have had any unprotected sex the delay in ur periods could be bcz of an unwanted pregnancy ,or it might be due to changes in diet pattern u may have had recently,sometimes lack of physical activity and weight gain also leads to delay in periods .Stress either physical or mental ,changes in life style is also sometimes responsible for irregular periods. Apart from this condition like PCOs , thyroid or any other harmonal imbalances,apart from pregnancy can also lead to a missed period.