Whats wrong with my elderly cat

I have a cat that is about 18 yrs old. Lately he has drooling excessively mostly from the left side, he's drinking more and sleeping more deeply, and he hangs his tounge out almost all the time to the front or the right. He has started to use the bathroom all over the house even when we take him to the litter box. He eats, but sometimes throws whole pieces back up. He has recently gone to the vet and his blood work was fine. Any idea on what could be happening?

Well. obviously he is not fine, you need to take him back to the vet.
   
Even a month is enough with an older cat to have a significant change in health. He could have an infection, tooth problems, kidney failure, heart trouble, the list is too long to go on.
   
Vet ASAP.