45 days late period with negative test ..backache and mild cramps what's happing ??

Irregular Menses: It is common for young women to occasionally have their periods last longer/shorter, come early/late or miss them all together. If you continue to have irregular cycles x 2 more months, see ur clinician for an evaluation. Cramps and backaches from prostaglandins - substances that cause uterus to contract to expel menstrual tissue when you do not conceive. Try naprosyn (naproxen) or ibuprofen.

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.