Pulmonary edema is the buildup of fluid in the within these air sacs leading to shortness of breath. Located in the lungs are thousands of air sacs (alveoli) which are small balloon-like structures where the exchange of carbon dioxide and oxygen occurs. Tiny capillaries surround the alveoli on the outside. When the cat breathes in, air enters the lungs, causing the alveoli to expand. Oxygen passes from the alveoli and into the capillaries, carbon dioxide from the capillaries pass into the alveoli and is exhaled out. The most common causes of pulmonary edema relate to problems with the heart, such as hypertrophic cardiomyopathy. Heart diseases can lead to reduced movement of blood through the capillaries in the lungs. As the blood flow slows, fluid leaks out of the vessels into the airways. 
Thank you for your email. Without examining Missy, I can't comment on what might be going on. She may just be a skittish cat, given all of the change that has gone on in her life recently. IF she is eating and drinking normally, and seems healthy otherwise, she may be okay. If she seems to be having breathing problems. it would be best to have her seen by your veterinarian, and they will be able to determine if she is normal, or has a problem. It is nice of you to help this cat when she needs it. I hope that she is okay.
I'm afraid that I can't offer much more assistance for Bella than what you have already done, without being able to see or examine her. If she has not had a trans-tracheal wash, that may be one test that can help determine the type of cells that are present in her lungs, and if the x-rays have not been evaluated by a specialist, that is another possibility, as your veterinarian should be able to send those x-rays to be evaluated further.
A stray cat showed up at my house a couple days ago. She is very thin and I've started her on small throughout the day. She is eating, drinking, voiding, and stooling okay. Eyes, ears, and nose are all clear. The only major thing I've noticed is rapid breathing when purring or stress (loud noises make her skittish). I have pets of my own and really cannot afford a costly vet visit, and our local shelter and a rescue are overwhelmed with companion animals at this time. I've started her on antibiotics and am keeping her isolated from my own pets. My daughter has asthma and this looks very similar. Any advice? I've found out that her previous owner passed away and a neighbour tried to bring her to her house, the cat bolted and they couldn't find her, 3 weeks later she showed up at my place. She is a spayed 5 year old female who was up to date on shots.
Queening is a painful process just like childbirth is for humans and some heavy breathing or panting may occur afterwards for a few days due to discomfort. But if you feel that she is straining or in distress you should visit your Veterinarian to check her over to ensure that there isn’t a retained kitten or another issue leading to these symptoms. Regards Dr Callum Turner DVM