Got my first yeast infection 3 months ago and i keep getting them every month, ive been to dr got medicine. what could cause this?

Hard to say: The yeast is a normal component of poop. The proximity of the vagina to the anus makes cross contamination easy. Yeast thrives on glucose, which is present in vaginal fluid & even higher amounts if you are diabetic or pre-diabetic. Toileting or sexual practices may make you more prone to recurrence. Yeast is always around, even in the dust, so you won't escape it.

Partner?: I agree with Dr Ferguson's assessment but would add that your sexual partner (esp if uncircumcised) may be carrying the yeast (candida) without any symptoms. If you keep your tampon too long during your period, you may be more at risk to develop a yeast infection. Consider changing it more frequently even if you were a light bleeder