Suggest Treatment For Irregular Menstruation

Hi, I always have my periods regularly but this time I had irregular spotting for more than a week just before periods, consulted the gynec and she prescribed Duoluton L.She asked me to do a 21 day course of this tab and should nt skip in between. I got confused when to start and just took one tablet. I didn't continue after first day or complete the course as I had my own doubts. Is it ok to start taking this tab any day if the periods are irregular this month. The spotting has not stopped yet, it is more than two weeks now..what should I do..