Sore boobs, brown discharge, lower backache, a week late period and a negative test? what's going on?

That brown discharge: might actually be your period. Reasons for missed /late periods: low body weight, obesity, marked weight gain/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 is pregnancy.