Hello, im 8month pregnant and while feeling sex i feel pain in my vagina from inside does this causes the babys water bag to burst before the due date?

No: If you're having pain with sex, i'd raise the point with your ob. Conveniently, you're seeing them nearly every week by this point. However, nothing you do from the outside like sex should affect the amniotic sac unless the sex is particularly "active" or you are inserting thin objects or chemicals. There are some positions that can cause pain or cause fetal distress, discuss these too.