Am I just starting my period late again for a reason?

Yes/no. If you have not had sexual contact i'd say no problem.Your cycle needs a little time (a few months) to settle in.
Don't know. 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.