The reputation I gained in a specific day passed from 115 to 90, as shown the next day. In the summary, I haven't found anything that could explain why the reputation was decreased.
How can I find the reason of the reputation lost?
Go to the reputation tab
Click the "show removed posts" checkbox at the bottom
You should see an account of wherever you lost rep due to deleted posts, etc (i.e, non-public stuff).
If it still doesn't add up, go to http://stackoverflow.com/reputation, scroll down, and select "Trigger Reputation Recalc". This isn't supposed to be needed now, but it doesn't hurt to try :)