If you kegged a 5 day old beer, that's pretty early! I've never kegged a beer younger than 2 weeks old, except for the one time we did a British mild swap and I kegged it at 8 days. Even after fermentation is over, the yeast are still busy, "cleaning up" their own waste products, such as diacetyl. After the fermentable sugars are gone, and FG is reached, the yeast start chowing down on diacetyl and other substances before going dormant when there is nothing left.
As far as temperature, were those temperature you gave the fermentation temperature, or the room temperature? Do you monitor the actual fermentation temperatures during primary?
My gut feeling is that you removed the beer from the yeast way too soon, so the yeast didn't finish the job of removing the diacetyl, and then kept it cold so that the yeast have gone totally dormant.
Not much you can do now to clean up the diacetyl, but you could try keeping it at 70 degrees for 2 weeks to see if the yeast remaining in suspension can clean it up a bit.