You hit the repcap today. Congratulations.
To fully explain what was going on with your reputation, I first need to explain how the repcap works. Note that a lot of this is covered in the Help Center, but I'm going to go through it in rather more detail than they do there.
The first thing to understand is that reputation changes broadly fall into two categories1:
"Standard" changes. This encompasses most activities; rep you gain when you receive upvotes, rep you lose when you give or receive downvotes, and reputation you gain from having suggested edits approved.
Bonus changes. This includes the association bonus, the accepted answer bonus (that is, the +15 you get when an answer of yours is accepted and the +2 bonus you get when you accept an answer on one of your questions), and any rep gained or lost from awarding or receiving a bounty.
With this in mind, the repcap requirements can be restated as:
Every day, you can earn a maximum of 200 reputation from standard changes.
With this in mind, let's look at what happened to you on December 22, 2015. I find it instructive to think of your reputation like a glass and bucket of water:
- Every time you gain reputation in a standard way, an amount of water is added to your glass
- Everytime you love reputation in a standard way, an amount is removed from your glass
- At the end of each day, the glass is emptied into a big bucket
- Any bonus changes affect the bucket, not the glass
- The glass has a finite capacity, and any overflow is lost forever
So looking at your reputation history for the day, we can reconstruct a timeline of what happened to your glass:
Skip ahead a bit to:
1 I'm excluding a few edge cases, like reputation changes to to retracted votes and deleted posts; those get complicated, and aren't useful here. I'm also excluding the hilarious "Arbitrary Reputation Change" category, which I believe has only been used on StackApps. Finally, there's a -100 reputation penalty if one of your posts receives 6 valid spam or abuse flags; I have no idea how that factors into the repcap, though I doubt it's something that comes up very often