How a naagin become a ku-sarp

We saw sesha as from ses vansh in season 1, then she become takshak nagin in season 2(which they explain), How did she suddenly become ku-sarp and got those abilities. We never saw her two headed abilities in season 1. Same goes for vishakha she was told as ses vansh nagin in season 3 then at the end of the season she is ku-sarp. Then we get to know in season 4, she is from kal-kuth vansh. Why are they changing character background and didn't gave any reason for this. Vishakha is suddenly using her ku-sarp abilities so often, then how she never used it in season 3 at all even when she is near her dead bed