10 days late for my menstrual. sever abdominal pain, painful to touch. took at home pregnancy test but results were negative. could this be serious?

For severe abdominal: Pain get med eval now. Possible reasons for late 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.