Missed period and not pregnant

So I haven't had my period yet at all this month. Went and got a blood test done and pregnancy is out the window. They said having a missed period is also called secondary amennoreah and it can be caused anywhere from stress to a number of reasons. My period has ALWAYS been an accurate 28 day cycle. I'm very concerned and worried considering this hasn't ever happened to my before unless I was pregnant. I have an apppintment scheduled with my OB. I'm very scared and worried and all I can do is cry. Has this ever happened to any one of you fellow ladies? If so, please tell me. I'm all ears and could really use any advice/tips that yall may have! Thank you in advance.