My cat was meowing at the front door, and the minute I let him in, I could tell he wasn't acting right. He went and laid down on the carpet immediately and began meowing I walked to the kitchen to give him some food, but he didnt come immediately like he usually does. When he finally decided to go for the food, he didn't stand up, instead he pulled himself to the bowl using his front paws, I was confused and thought he was just acting weird so I let him eat. He drank almost his whole bowl of water (which is also unusual for him) and then came to lay on the bed with me. He tried jumping up onto my bed, but couldn't make it, it's like his back legs weren't working. When he lay next to me I examined his legs and found a sore on his right one, but I don't see how something that small could affect him so much. He got up and walked to the bathroom, and he wasn't limping or walking funny anymore. When I tried looking at his leg again, he wouldn't even let me come near him, he's never bitten me or attacked anything / anyone the whole time I've had him, but when I sat next to him, he hissed at me and looked like he was about to attack. I let him be and decided to just keep an eye on him without getting too close. Once he had fallen asleep I saw he was shaking/shivering and even twitching, his heart rate increases tremendously and was restless in his sleep, too the point where it looked like he was a having a nightmare. I stoked him calmly which calmed him down slightly, but he would still shake and move around alot. It's now been 4 hours since he came home and he's hardly responsive, he sometimes looks squint and shows his 3rd eyelid. When he tries to walk he stumbles and can barely pick himself up. I don't know what could have caused this, there are other cats around in the complex, but I've never seen them fight, and he doesn't have a big enough wound to be so sensitive on his leg. There aren't any dogs around here, but he could have run into a stray which may have attacked, or could a snake have bitten him? or has a disease / illness caused this?