Sorry for the problem with your cat. I had a similar problem with a horse, some times the seditive can have a odd effect as they are coming off of it. My horse, we believe had eplicitic (msp) siezures, but we did not know that at the time. She did live but it was scary. After that you could tell when she was going to sieze. Not a pretty sight. She couldn't be trusted in the show ring, trailer, anything that had stress. Not even as a broodmare.
Hope the poor cat comes out alright. It is sad when we can't help them. :(