What Causes Missed Periods Despite Having Negative UPT?

My husband and I decided to try to conceive as of last August. I stopped taking birth control in February of last year (pill). I got my period at the end of August as scheduled, but have not had another since (now four months overdue). I have taken many pregnancy tests but all have come back negative. I have had no other noticeable problems or symptoms. I went and talked to my doctor and did the required blood and urine tests. My doctor also had me do an ultrasound (intrusive) and I did. That was over a month ago. My doctor is on vacation until the end of the month so I will not know any results until after then. I am 30 years old, no prior health problems, no prior conceptions or miscarriages, and no known family medical history. What could be the reason for missing so many periods consecutively? I have missed one here and there in the past, but never more than one in a row and now I am very concerned.