Period is 4 days late, experiencing lower back pain, took 2 at home pregnancy tests and both read negative...what could be going on here?

Repeat HPT in 1 week: if still no period. 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.