What Causes Irregular Periods After Taking Unwanted 72?

hi.. I took unwanted 72 tablet within time. I did this same twice. and after that I was expecting my periods on 26th feb but I got on 20th feb. but it was just few drops and then stopped. I was worried so I tried pregnancy test and even that shows that I am not pregnant. kindly guide me what to do to get my menstrual cycle back properly. awaiting for your guidance. thanks