What are other reasons for a missed period other than pregnancy?

PCOS: If you've just missed one period and it doesn't happen often, then the most likely cause is probably stress. The most common cause of frequent missed periods is polycystic ovarian syndrome. The normal hormonal cycle is dysregulated, ovulation doesn't happen and your periods become very irregular. Pcos is associated with a lot of other metabolic problems, so it's best to be checked by your doc.

Anovulation: If you missed a period and your pregnancy test came back negative, then you most likely had an anovulatory cycle, meaning your ovaries didn't release an egg. This happens from time to time but if consistent, see your doctor because you may have a hormonal imbalance. Consider a birth control consultation to regulate your periods and avoid unplanned pregnancy.