I have my missed period, frequent urination,white discharge but negative pregnancy test. what could be the problem?

Infection: You may have a bladder or vaginal infection. I would see your doctor to get a definitive diagnosis

Vaginitis: White vaginal discharge often indicates a vaginal infection which can also irritate the urethra and cause urinary frequency. Of course a urinary tract infection causes urinary frequency as well. A urine culture and colony count is ordered to check for a uti. A gyn exam checks for a vaginal infection. Repeat the pregnancy test to double check since you missed your period.

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.