I had reputation of 104 it fell down to 96 the next time I logged in. Why is that ?
There are several reasons why reputation can go down (some of them mentioned in the other answer), let me complement and expand that a bit:
If a post gets deleted (by the system or users), then typically the reputation gained from that post is removed (there are exceptions for posts on the site, since a longer time and with good score). It is quite likely you were affected by this, and Asaf explains how to find out.
If one of your posts get down-voted you lose two points (except it was in CW mode). However, this would show on your reputation page.
If one down-votes answers you also lose points a point, but you cannot yet down-vote.
If a user gets deleted their votes are invalidated, so if a now deleted user voted for you this could result in loss of point. However, this would also show in your reputation page (user was removed). There are also other scenarios in which votes are invalidated, but again this would show (as voting correct).
If a user un-accepts an answer the answerer also loses the points. But again this would show in the reputation page.
Long story short: go to the reputation page and check the mark "show removed posts" Typically you will find the reason there, and it should be the first one.
It may be due to a variety of reasons-
- User was removed-Let you have posted an answer to User A's question and earned some reputation.User A was removed due to some reason.
- You downvoted something.
- Your question was downvoted.
- You deleted a question in which you had gained some reputation.