Why am I missing my period

I went off birth control in June 2014 and I got my period without question every 37 days. In January I missed a period, took 5 at home tests and went to my doctor for a blood test and all were negative. Now I have missed my period for March. I'm trying not to get my hopes up but it's so hard! Why am I missing periods, I don't feel stressed, nothing in my life has changed, but my periods just dont come. My doctor said not to worry until it's been a year of irregularity but why was I do normal and after 7 months it's not normal anymore! Any advice or help is greatly appreciated!