I need help..

I know this isn't the right page to be asking this.. but my dog has a tumour or a sist or something in his chest the size of an apple.. and it's so hard like it's ready to burst. the vet said its cancer and  since the tumour blew up to this size so quickly he only had about a week or two to live.. but what I don't understand is why he's so happy, he's still eating/drinking, he plays with my other dog,  and also can still walk/run perfectly..  do you think the vet was wrong ? he looks clmpletely healthy to me..