That's an interesting question and while I can't say for sure, there are a few things you should have your veterinarian look at. First of all, it's uncommon for cats to open their mouths when the breathe. This usually indicates something seriously wrong that needs immediate attention. Respiratory infections can lead to labored breathing but these are usually accompanied by other signs of disease like lack of appetite and fever. In your case I might ask the veterinarian to listen to my cat's heart with a stethoscope. Given your cat's relatively young age she might have a developmental abnormality involving the heart. Since the heart is key to pumping oxygen rich blood to the tissues, poor heart function can show up as labored breathing. Get her in for a physical exam and get to the bottom of this problem.