What Causes Vaginal Thrush?

hi i'm 37 years old women my problem is from long time i having a vaginal thrush.and discharging some thing with itching. this happening twise a every month.my i.p cheked my everying and gave tabs allso but it is still the same.still i trying for a nother baby from long time but i fed up this happening.please let me know why this happening.