I've missed my period for two months now. this is the first time it's been consecutive. i'm not pregnant, 21 and fairly healthy. is this serious?

Reasons for missed: Period: low body wt, obesity, weight ^ or v, ^exercise, endurance athletics, breast-feeding, stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, perimenopause, just starting menstruation, hormone irregularities, drug use, meds (i.e., birth control) or medical problems. #1 reason is pregnancy. See dr if no period x 3 cycles.