What is wrong with my cat?!?

Sounds like he hurt his paw. Maybe he ripped out a claw? Check to see if he is favoring his paw at all.

Something is going on with your kitty, but you already knew that.

Cats usually hide when they are in pain or sick. In the animal world, weakness means death, so many animals learn to hide when they do not feel well, to avoid becoming a target for other animals. Your kitty is lucky that he is living at your house and can hide in a safe place instead of being outside.

I would recommend taking your cat to the vet right away to rule out any physical causes that could be making him hide and hiss at you. He is getting into the 'senior' cat years, so I would make an appointment with your vet.

The only other reason I can think of that would cause your cat to hide and hiss at you is that you smelled funny to him. Did you alter your routine at all? Go somewhere you don't normally go? Change perfume? Cats are very sensitive to smells and are very habit driven creatures, and if something isn't right in their world, you will know about it!

My own kitties would smell me all over every time I came home and if I changed something, like soap, lotion or laundry detergent, I wouldn't hear the end of it until I switched back.

This could be so many things. I would take him to the vet -- it could be something really serious, like a blockage in his intestine (This is something you need to find out right away, if so. If his intestines burst, he will die :( ) He could also be freaked out for some reason-- a loud noise, roadwork, a cat fight.

I would suggest a visit to the vet immediately and that you pay more attention to what is going on around him to find out, since he can't just tell you with words.

Was he stuck in the closet the whole time? Do you have other pets? He may have gotten in a fight with a raccoon or something if he is an outdoor kitty. But right now, be as nice to your kitty as you can be. Clean his litter box often, make sure he has clean water and always has food, and pet him and give him his favorite pillow. Eventually he should cool down.