Don't know what's going on!!!!

So I haven't had my period since June and I'm 20 I've always been regular until like I said June. I've gone to the dr and have taken pregnancy test and all test including blood tests come back negative. I can't get my dr to answer my questions or help me understand. It's been 4 months now and no period has anyone ever experienced this and not been pregnant? It's really freaking me out!