1 Women who suffer from anorexia may also have erratic periods.  The menstrual cycle varies from woman to woman. While we say that the average menstrual cycle is 28 days long, that is not true for everyone. Sometimes our period is believed to be late when in all actuality we have simply miscalculated. If you have irregular menstrual cycles but know when you ovulate, look for your period about two weeks after you ovulate.