The more literal usage of the word "douche" refers to something clean and good (a vaginal cleaner). Yet to be called a douche is much more negative. Also can be used as a negative term in other forms, much more common as negative slang than the literal meaning. Similar to the way that a "fuck" is a good thing, but it's never good to be called one ("hey why'd you do that you stupid fuck!?") or to be fucked over, etc.
To put it in perspective:
In the more literal sense of the word:
When I was younger my priest douched me to clean out my evil spirits.
In the slangier, negative sense:
Now that I'm older and undergoing therapy I realize my priest really douched me over!