Missed periods but not pregnant (PLEASE HELP)

So I haven’t had a period in two months!! I know for a fact I’m not pregnant, I haven’t had sex in over a year and I went to take a pregnancy test at the walk in clinic. Not pregnant. I have no health issues, I got a blood test done and nothing alarming came back. My diet hasn’t changed, etc. I’m only 20, so age isn’t a factor. Absolutely nothing is different since my last period. I’m wondering what could possibly be wrong, I’m getting SUPER concerned. Does anyone have any possible theories before I give in and go to the OBGYN?