There's really no way of knowing if this will help or not, so I'll just throw it out there... Louie's mysterious episodes are baffling, for sure!
Last winter, on a couple of different occasions, Willie suddenly yelped (more of a scream, actually), and then ran and hid, trembling uncontrollably. It was awful. I was sure he was in pain, and so rushed him to the Vet each time. At first we all thought he might have hurt his back, but then his doctor could find nothing wrong with him physically. Nothing at all.
So I'll cut to the chase... Willie had received a couple of jolts of static electricity. The air was kind of dry, and I don't have a humidifier in the house. Anyone who has ever gotten a shock from static electricity knows how much they hurt!! It took us a while to figure this out. The trembling was really from fear rather than pain. I couldn't explain it to him, and even if I could, he still wouldn't like it!
My remedies for this are ongoing... In the winter when the air gets drier, I replace all plush, polyester blankets with cotton ones. I replace his stainless steel dishes with glass ones. These things are much less likely to generate static electricity. Also, I buy a fresh spray can of "Static Guard" and spray it around places he likes to hang out. I spray his sweaters, too. When I wash his bedding, I use unscented fabric softener, which reduces static cling. These measures have helped a lot. Don't know if that's what has happened to Louie, though.
Good luck with Louie!!