Yesterday I saw my reputation as 1024. I didn't have any activity on Stack Overflow. But today it shows reputation as 910. What is reason for this?
Probably, one of your post got flagged as spam/offensive, check at http://stackoverflow.com/reputation and look for -100 post
Quick calculations on your posts is like this
Now you have 910, so you are losing around 100, so most likely it was because of a post, which got flagged as spam/offensive (6 flags needed to be happened)