Back pain between the shoulder blades. sore outside breast. period late.what can this mean?

Unsure: First, always do a pregnancy test when your period is late. If your pregnancy test is negative, then I can't give a good assessment without more information.

Uncertain . Take HPT: Reasons for missed /late periods: low body weight, obesity, marked weight gain or loss, over-exercise such as in endurance athletes, breast-feeding, stress, illness, eating disorders (bulimia or anorexia), travel, hormone irregularities, drug use, medications (i.e., birth control) or med problem (polycystic ovary syndrome or Asherman’s syndrome. #1 reason for missed period is pregnancy.