If your fat percent dives way down you can lose your period, so if you have been dieting or exercising and are very lean that could explain it.
PCOS is another possibility. PCOS seems to be triggered by a sugar/insulin problem, but I don’t believe the relationship is proven yet. It is commonly accepted by reproductive endocrinologists that it does affect ovulation, and cycles.
Which brings me to pituatary or thyroid problems. Do you have an discharge from your nipples? Has your skin been very dry, sleeping habits really screwed up, dry brittle hair that might be falling out more, dry eyes?