I just noticed that I received a "User removed" message from The Workplace last week, and my rep is reduced by 15. The funny part of this is that I haven't visited this section for few weeks.
So may I know what is going on?
This isn't from you being removed. Some other user was removed. When a user is removed, all of the votes cast by that user go away. In this case, it seems a user was removed who cast upvotes on your posts (three upvotes on questions is +15, or one answer and one question). Since these votes have gone away, the reputation you gained from them has been lost as well.