Thanks Jim. After checking a few headers and confirming that spam assassin wasn't working, I emailed firstname.lastname@example.org explained the situation and asked that spam assassin be restarted on my server, Danzig. In little over an hour I received a reply saying that this had been done, and now my incoming messages are being filtered once again.
Yesterday morning after I first logged on at 9 a.m. I couldn't access my mail or my website for well over an hour, so I suspect Danzig must have taken a hit of some sort, or perhaps been rebooted. Because yesterday is when the spam started flowing in...but I'd forgotten about checking the header. it's easy to forget now that I rarely have to deal with it anymore!