I think you need to get a second opinion right away, something is clearly going on. With IVDD you can sometimes treat it with rest and medication if you catch it before the disk gets too bad. If it were my dog, I would crate him 24/7 until you can get him checked by another vet (the sooner the better, in my nonmedical opinion). In the crate 24/7, only out to potty, and you carry him to potty and carry him back indoors. No stairs, no jumping off of furniture. Best to keep him on a leash while he is pottying to keep walking to a minimum. I think you'd want a harness, not a collar, just to be safe. If your vet does not think anything is going on, I would ask for a referral to a neurologist. If the answer is no, try another vet, I think you are right to be concerned. The symptoms do not sound exactly like anything I have heard, but they could possibly be IVDD as you say. Even if it is "just" back spasms, you want to find out what's causing them. I am sorry your vet kind of passed it off, but I think you need to trust your instincts. You have seen these episodes. If you see another episode and can possibly do a video, that might help when talking to the vet. You are right, shih tzus are one of the breeds that get IVDD.