If you had really low body fat, that would be an explanation. Also, stress can cause women to lose their periods -- I've heard of someone in college who only had periods when she was home on summer vacation. But if you don't have a super low body-fat level and don't feel particularly stressed, I'd check in with a doctor (ob-gyn and / or endocrinologist) and ask for some hormone levels to be run, an ultrasound, maybe a thyroid test. If you're fairly healthy and moderately active, it doesn't sound like anything is really wrong, but your periods are AWOL and you should be able to get help finding out why.
I agree with anniebrooke's comments. To add, you probably should tell your mom. Doctors generally want you to have a period at least every three months. They may also test your hormone levels, your thyroid, etc. Thyroid issues can make you miss periods too. I had this issue in my early 20's so it is not just something that happens when you are older. Start with your GP or OB/gyn who can draw blood for these simple tests. good luck
(My answer above assumed that you saying "I have never been sexually active" meant nothing of any kind that might get sperm near the vagina. Just in case you've done a few things that are sort of close, even if they do not involved penetrative sex, I thought I'd mention that there are cases where women have gotten pregnant without the guy's penis being in their vagina. If you think there is any risk of that, also get a pregnancy test when you're at the doc.)