My beloved golden, Agnes, died of lung cancer in May. Her increased respiration was the sign. Apparently, her body adapted to the cancer and until it spread to her lungs, there were no signs that she was sick. The end was fast, 10 days during which her breathing became increasingly more labored. I'll never get over losing her--a very special soul.

If your sweetie's always had fast respiration and the vet says she's healthy, I wouldn't worry. Taking her in for a lung xray will alleviate your concerns, however.