What Causes Irregular Periods?

hie doctor..my age is 21,weight is 50,and the height is 5:4. i am getting delayed in periods from one month.i have been satrted with this problem before one year.i have consulted to doctor for this she told me to take a medicines for 4 month regularly,i used to take them,after that it was alright.Iwas getting my periods properly,then again the problem has been started.tell me whats the reason behind that