I always get bv after my periods, what causes it and how can i avoid it in future without taking medication.

Whoever discovers it: She who discovers how to prevent bacterial vaginosis likely will win a Nobel prize. BV happens to women everywhere. A woman may do better if she uses condoms to keep semen out of her vagina, to avoid more stuff with high pH in there. Current goal is to keep a lower pH to nurture "normal" vaginal living microbes. Good webpage: https://www.uptodate.com/contents/bacterial-vaginosis-beyond-the-basics