Before few minutes I was having Reputation 169 ( on Stack Overflow ), which I got by Edit Approval. Now it is showing me 165.
What could be the reason behind this ?
If you have any questions or answers, it may be that you picked up a couple down-votes. If not, something else you picked up positive reputation from probably got deleted.
In any event, the system isn't broken. In time if you continue to make quality contributions, your rep will go up even if you see small setbacks from deletions etc. Don't sweat it. Analyze it and see if you can learn anything, even if that is what is so low quality that it isn't worth salvaging, then move on to positive contributions.