What Causes Irregular Periods After Taking Birth Control Pills?

hi , My name is chitra 25 yrs, height 5.2, weight 68 kgs married . I had my regular periods every months . I had coppper T inserted on 20/5/10 . I had my regular periods from then onwards. but from january 20/1/11 my periods were not regular . so i went to the doctor on 21/3/2011. she checked & told me that I am fine . she gave me Meprate tablet to take 1-0-1 (10 tablets). then i had my periods on 29/3/11. now again i have not had periods till this date 10/6/11.now what is my problem