One of the kitten (about 3 week old) that my husband saved from a construction site in Hong Kong grew up with heavy breathing.
When my husband found her, she was full of cement and had probably been eating cement. Her eyes were stuck closed which had to be forced opened and we found that one of her eye was damaged by cement, she pooed cement, and her fleas were sooooo big. I was told that she had to be put down 'coz she won't live, the vets will not even give her a single drop of "Frontline".
Decided to take her home and my husband gave her one drop of "Frontline", put her in our computer room with her own bed, food, water and a pooh tray...she's now 3 years old.
Her heavy breathing is obviously caused by all the unfortunate tragedy she had before.
Back to your question, get your vet to look at her. If there are no other problems that occur due to her breathing (i.e. weakness) I honestly think she is fine.