How does “Reputation” work?
My rep has decreased from 3189 to 3144 suddenly and when I checked I saw that one of my answers has been deleted that had 45 votes on it. How is that possible? Is that a mistake or something else?
The entire question was deleted because it was asking for a recommendation for a blogging engine. That's not a programming question, so it was rightly closed then deleted by the community.
The question to which your answer was posted probably got deleted.
That would remove your answer along with the question, and automatically deduct the reputation from your score. You don't earn reputation for deleted answers. (In general; there are specific exceptions.)