All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
You are not obliged to wash between your fingers when washing your hands after Istinjaa' (which is to remove what comes out of the two private parts from the place where it comes out or the place that is nearest to it, whether by washing or by wiping it with stones and the like). This is because, in principle, the area between the fingers is pure and you are not obliged to wash it or your clothes.
You should avoid giving in to the whispers [of your soul or Satan], as this is a widespread evil that could even render a person insane. Moreover, you should know that in principle, all things are pure and not impure and if you doubt that a pure item has become impure, then in principle it is still pure and not impure.
Allaah Knows best.