13 days late, cramps, clear discharge, negative pregnancy test?

You propably: Did not ovulate.If it happened again this month i would advise that you call your doc. You can go safely 3 months without a period not on pills but should be seen for sure at that point.

Missed 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.