What might be some causes of a late period?

Depends on how late: There is a difference between being later than expected and abnormal. The menstral cycle can very from 21-36 days so whats late to you may be considered normal medically.

Late period: 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. #1 reason is pregnancy. Take a pregnancy test if you are late.