I've lost 5 units of reputation. How I can find out why?
There's been lots of discussion on this. Losing 5 points in one shot is probably a reversal of a question upvote, and the problem is that such reversals merely erase the previous upvote, so you can't track it. One thing you can do though is look at cstheory.stackexchange.com/reputation once you're logged in. It gives you the entire history of points.