RAPID weight loss

[iplaay]

12 yr old siamese, 5 lbs in 3weeks, eating wet food, shiny coat, nose cold, drinking water, maybe a little more than usual, muscles in hind quarters are almost gone, sits or sleeps a lot, can still walk, sits often. spine and hip bones are sticking out. Not complaining, but getting thinner by the minute.
HELP!!!!!!!!!!!!!!!!! We had blood tests done, high white cell count, tried antibotics for one week, didn't help. Vet says probably cancer, but not sure.