I am sorry to hear about this stray cat.
If you look at her gums, are they pale, or do they look a normal bright pink?
In a cat this young, with labored breathing, I would be worried about anemia (low red blood cells) which in turn could be caused by the feline leukemia virus. If her gums appear pale, that would be the first thing I would check for, before doing other tests.
If your area has an animal shelter with a clinic, you might have her treated there.
If there is no low cost clinic in your area, you can check this website for places that help with vet bills:
But before dong all those tests, I would do a simple blood test to see if she has feline leukemia.
Let me know what other questions you have.