Missed period but really bad cramping what can it be?

Menstrual cycles: It is common for young women to skip a period or have irregular cycles. It is also more likely to happen if you are very athletic, gain or lose weight, using birth control pills, or have an eating disorder. Still, if you are sexually active, you could be pregnant. I would track your cycles to see if it happens often and see your doctor if you notice that it becomes more irregular.

Uncertain: 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.