I Have Had 30 Days Of Regular Till The Month

I have had 30 days of regular till the month of June, my period was late by 5 days and then July it was back to normal 30 days. But I have missed my period for both Aug and Sept. When I did hpt last time it showed neg. So went to doc. I had heartburn and white discharge throughout the time. So I was tested for hormonal changes but everything came back alright. I have never gone this late without a period. And I'm not stressed other than the fact that my periods have gone missing. What could be the reason?