Yesterday I noticed that my reputation had decreased from 210 to 132. What could have caused this? I checked my activity and didn't see a decrease in any of the data.
Your rep was recalculated yesterday (27th). One of your answers was on a question that was deleted (question about slicing a pizza in to 6). Your answer was upvoted 7 times, so maybe it was a recalculation based on the now deleted answer.
I'm new to this though, so I'm kind of guessing. I do know that rep is recalculated based on changes in votes, changing acceptance, and answers or questions being deleted.