to add about the Feline Leukemia...this also can cause these symptoms, however most cats do not live as long as your with this virus. Also, it your cat has ever been vaccinated for the virus and not really exposed to the virus, then its very unlikely the cause of your kitties problem.
18 years old a pretty up there in age. There are a multitude of possibilities. Coming to mind with a cat this age could be kidney failure. If the electrolytes are way out of balance (which happens with extreme kidney failure) seizures result. Epilepsy is also possible in which medication can be used to control seizures. In terms of cancer if its the case, I would be concerned with brain or spinal cancer. Unlike stated above, cancer is NOT contagious, not even to other cats. There is a condition called Feline Leukemia which is confusing because humans have a cancer called Leukemia. However, Feline Leukemia is NOT a cancer. It is a retrovirus that attacks lymphocytes. Due to the attack on lymphcytes is ACTS like a cancer...but the point is it is a retrovirus. Being a virus is why it is contagious to other cats.
she is a very old cat but my cat had cancer and would throw fits loose weight and stay far away from us if a cat has cancer unlike humans they can spread it to other cats plz get this kitty checked out
sounds very much like she is having seizures....this kitty really needs to see a Vet asap...there are medications to control this.
she needs some help jonno!!!!