What Causes Irregular Periods?

Hi i am 16ears old girl. I am always having irregular menstruation cycle and periods always occur two days before the exact date but this time my date has been passed and i havent got periods till now.. My date was 21st february. I also took PONSTAN (PILLS) during my menstruation period last month for the first time to decrease the flow of periods..... Have that effected my menstruation cycle this month??? thats why my periods are late??? and i m also having white discharge whole month.. Are my problems the serious one? how can i fix them?