It's Feline Asthma! The good news about this is that asthma is treatable and Penelope won't die. Phew!
The bad news is that Penelope will have to take medication every day for the rest of her life and it's pretty expensive (about $100 a month), but, honestly, it seems a small price to pay to keep my little girl happy and healthy: I'm just so relieved we have a diagnosis and that I can do something about her coughing/wheezing attacks.
I went to the vet today to pick up Penelope's new Feline Nebulizer (called Aerokat). Here's a picture of it: http://www.aerokat.com/overview.htm
Then I had to go to the human pharmacy to get the prescription for Flovent filled. The pharmacist was very surprised to be filling a prescription for a cat and wondered how I was going to administer it!
I tried to administer a dose of the Flovent when I got back and Penelope was not too cooperative but I think she got a few huffs of the medication. I think we both need practice using the Aerokat!
Anyway, thank you all SOOOOO MUCH for your support. It really helped me through a very difficult time. You all are the best!