I Haven't Had A Period In About 4 Months. I

I haven't had a period in about 4 months. I thought I was pregnant and went to a obgyn for a test. The test was negative but they did not want to do any more tests at that point. It has been another two weeks and I still haven't had a period. What could be causing this? I'm not stressed and nothing has majorly changed just kinda confused.