Feline lung cancer symptoms?

My 12yr old cat has become breathless & since yesterday her left leg, chest & throat has swollen. The emergency vet said she may have a bad heart but she thinks its lung cancer. Friday another vet said she might have hyperthyroidism (there was no swelling then). Thing is everytime I look for symptoms they don't all add up. My cat is still eating, cleaning, going to the toilet ok (no diarrhea no vomiting no coughing no lameness) She's so not fazed at the moment, just breathless & swollen. Am so worried cos no matter what it is I don't want her to suffer.