the appropriate thing to do is to mention it to a mod and they will investigate and take appropriate action....what you dont really want to do is start a big stink about the rep system because it was taken away for a few days a while back due to abuse and other problems/complaints with it
First, I'd recommend treating all members with respect. Hence "loser" isn't an appropriate way to complain, nor is it mature.
Second, their feedback may be legitimate. What you need to do is contact a mod and let us investigate your post and their feedback. Just because you may not agree with what they said, doesn't mean it's not legitimate. However, if the feedback is completely uncalled for, then we will remove it.
To be honest, I just checked your feedback and the post in question, I see nothing wrong with the negative rep the poster gave you. It was neither insulting and it gave a constructive reason why they gave it to you.
I can see how someone would disagree with your post. Just because you receive a negative rep here or there is not the end of the world.