Can physical stress really cause a late period?

Yes, though: I'm not sure what you mean by physical stress. But anything that stresses the body---lack of sleep, poor nutrition, severe or chronic emotional stress, a trauma, etc., can disrupt the body's normal patterns of hormone secretion and cause a late period.

Yes: 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 (i.e. PCOS). #1 reason is pregnancy.