I have a feeling that the cancer has moved onto his brain, there is the scene with him going in to the catscan in the last episode and thats the first time I think hes been back to see the doctor since before he killed Gus, even Gus questioned Walt's actions and made that remark asking if Walt had returned to the doctor to be reevaluated after he murdered the 2 dealers. It could really explain Walts extreme character development after season 3.