So, I login today and notice that I have gone from 11,481 down to 11,195. I don;t see how this could have happened through the voting system. I know that another member was annoyed when I corrected their response in a comment and proceeded to arbitrarily downvote many of my responses, but that could not account for 286 points. It's not the end of the world here, but it is a little discouraging.
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Given that your reputation graph doesn't show a sudden drop, then chances are that a reputation recalc was done on your account.
This may have been triggered by the abusive downvoting you experienced. When those downvotes are deleted, a rep recalc is done to reflect the deleted votes.
In most cases a rep recalc results in a lower rep due to migrated and deleted posts.