Can there be any reason for a missed period besides pregnancy?

Yes there are: Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (polycystic ovary syndrome or asherman’s syndrome). #1 reason is pregnancy.

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.