Pls help! light period with mucus and backache, and its a late period (11 days) took 3 pregnant test & came out neg. am i pregnant? or any other reason?

Possibly: Pregnancy is always a possibility. Obtain a home pregnancy test. They usually become positive at about the same time the next expected period. If it's negative, repeat it in a week.

Late periods: 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.