My answer got me -15 points. Was the OP booted off the site, or what?
I am just wondering.
The user accepted your answer on Jan 1 at 22:15 (UK time), giving you +15 points, then today they changed their mind and unaccepted the answer so you got a -15 point reversal. Overall your net rep became just the +10 from the single upvote.
If you look in your reputation you'll see the -15 reversal is labelled unaccept.