I would also be inclined to think it was because of her weight. How much does she weigh? Is she able to clean herself ok? My Marius was so overweight at one time that he couldn't clean properly, so that would be an indication. Your vet would probably have the best view on this since he could listen to her breathing. I sure hope it's nothing serious! Get that precious girl to the vet and let us know what happens!
**hugs**
Karen