Why didn't i get my period?

i usually get my period on the 11th of the month. i didnt get it in September or October. so around a week and a half ago i started getting cramps so i decided to start taking birth control bills. but now my cramps are so painfull. and i fell bloated and im getting all the signs but i still havent gotten my period. this happened to me before. but i wanna know why this happens to me so much. my doctor says its normal. should i change my doctor. please tell me what to do???