Each cat responds differently in my experience, too. Some seem to feel fine, but want NOTHING to do with me for a day or two. I became a "traitor," and they have to decide when I am forgiven, ha ha. Others do react to the shot, and sleep or get sluggish for a 24 period. Others just don't want me touching anywhere behind their shoulders for a few days. I don't know if that is because the shot site is sore, or because they are wary of another shot coming along! And it is not just the shots, but also the stress of the vet visit, the carrier experience, the ride in the car. All that stress can tire them out too. So there is lot going on and it normally passes in a day or so. Good luck and call your vet if you are concerned!