I am currently 13 days late and assume I will miss my period for this month but why? I travelled to a couple of countries in the past 3 weeks but they are not too far usually the time difference is +- an hour. But I did get my schedule mixed up I would stay up all night and stuff. Also I caught a terrible cold around the time my period was scheduled or a couple of days before. So generally I never get my periods on the same day of each month, but that can be because of miscalculation on my part. So I'm used to being off by a couple of days or a week. Is this normal? Also I'm not sexually active.