For the last three months right before i start my period i get yeast infection like symptoms(itching and discharge).what could it be?

Vaginal discharge: Any new, abnormal, or unexplained vaginal discharge needs professional evaluation. At age 18, STDs (gonorrhea, chlamydia, trichomonas) are good possibilities, assuming you are sexually active. Yeast also is possible, but yeast infections usually cause itching and irritation with not much discharge. See a doctor ASAP. Good luck!