Not menstrating

We decided back in April that we wanted to try and get pregnant. I finished my last pack of birth control the last week of that month. I was having my period every 28-29 days with the exception of my first month of birth control. I haven't had a period since the middle of September. I've had two blood pregnancy tests that both are negative. Has anyone else experienced this? I've never missed a period ever before this. (My doctor says to wait another 30 days and check again)