What Causes Irregularities In Periods?

Dear Doctor, i am Ms. Mary Swapna(F), aged 22. i am suffering from irregular periods since 2yrs. i have consulted Dr. Meenakshi & Dr. Nagalakshmi @ Shekar Hospital, Jayanagar, Bangalore. I do have all the scanning reports, which says i dont have any problem. but still i am missing my regular periods. if i take tablets, i get periods if not NOOOO. PLZzZZZZZZZZZZZZZz give me a solution