Absent period 4 months. negative urine &a blood pregnancy tests. neg pcos. doctors won't investigate. could it be cancer? does cancer stop periods?

Cancer unlikely: Cancer would not cause lack of periods, expect in advanced case with extreme wasting. You may wish to consult a gynecologist if your doctor does not provide a satisfactory answer.

Should investigate.: If you've had regular periods and they've stopped suddenly (secondary amenorrhea), you need an evaluation. If your doctors won't investigate (if they're dismissive of your concerns), then find new doctors. I wouldn't think cancer first and foremost in this context, but you do need an answer.