I just lost exactly one thousand rep. I don't know why I look on my rep page and saw no reason, in fact, I should've gained a couple points. I know this question has been asked before, but I feel I should have been given a reason. Also please don't tell me that "it's not a big deal it's only rep" at the point I lose a privilege that I use (I actually use the being able to tell the amount of individual upvotes and downvotes a lot), it matters. So what happened and why?
Your rep was recalculated, it happens all the time on Area 51. You had points from proposal(s) that have been deleted, those points do not exist anymore, so they don't show up after recalculation.
You are currently at 263 Rep, so if you lost a 1,000 you probably had gained most or all of it from one proposal. I assume you were aware that the proposal had not moved forward and maybe that it had already been deleted.
I have lost as many as a couple thousand rep in one night. After the first couple of times you get used to it.
If this happened on a site other than Area 51, you normally get a pop-up that says something like -### Rep, user was removed