21 years old and never had a period?

I am 21 years old and I've never had a period. My mom took me to the doctor when I was 16, and the doctors ran some tests. My mom, never told me the name of what I have just that it was normal for me and I was perfectly healthy, but you know that look when someone is lying to you, well my mom has that look on her face every time we talk, it has really dampened our relationship. I am concerned that there is something terribly wrong with me and that I'm dying but my mom doesn't have the heart to tell me. Every thing I looked up about not having a period doesn't apply to me, I'm 5'6 and I weigh 115 lbs, I'm not under weight or over weight, I don't have PCOS, my doctor checked for that, I'm so confused, and I'm really getting worried that I have something new and no one has the heart to tell me they don't know what it is. Have you experienced this before, will I ever know what is wrong with me?