May be I've been on hallucination, but a few hours ago my rep on SO was slightly above 3000 (I even remember getting a message about being able to vote for closing/reopening questions) yet now I found it being 2800-something but I don't see the decrease on stackoverflow/reputation. How is this possible? Is this a bug?
Your reputation has gone through a recalc process and hence it has recalculated your reputation completely. You would have lost reputation if you have deleted some questions or answers on which you had got upvotes.
Check the answer in this link - How often is reputation recalculated?
Changes like this will now appear in your user profile, these changes will roll out to Stack Overflow soon (we're porting a few more things to the new data structures), it's only live on meta for the moment.