I have a yeast infection, my period is late, had unprotected sex about 3 weeks ago but got a negative pregnancy result. why is my period late?

May repeat: pregnancy test and see your doctor for treatment of yeast infection and get checked for other causes of delayed periods beside pregnancy

Don't know.: 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.