Period not coming what should I do?

I started my period at age 9 and when I was 15 they became very weird. It wouldnt come fir months and then it would come back. Im 16 now and it is still like that. Should I be worried? I went to one doctor and she told me since I started so young and I shouldnt worry. Also there is no way I am pregnant since I am not sexually active. What should I do?