I just now saw this thread. Is it possible that eating a few bites of food was causing poor Shredder even more discomfort? I am sending good thoughts his way. Hissy, I also just now noticed you have typed "Twilight Zone" in as your location, you have gone through soooo much since I joined TCS almost a year ago. I hope everything gets better really soon.
This sounds like what one of my cats had years ago: and abcess on her spine. She denned up and was tender at one spot, on her back. A shot of antibiotics and cortisone cleared it up, with in 24 hours.
The abcess was between the skin and vertebra and not visible, externally.
I hope Shredder is back to his old tricks by the time you read this in the morning. Here's a prayer for him, even though the vet said he seems to be relatively ok.
Poor Shredder and poor you! It must be so stressful, not knowing what is wrong... it's the one time I wish cats could talk! Healing thoughts to you both!