Find out if they did a Lab panel.
I have a ~15 year old, 20 Lb male cat. He was coughing and seemed to lack "wind" ...walk a few step and lay down, etc. He was normal in all other respects (eat, dump, drink, pee, play...follow me around, etc)
I took him to the vet, they did X-Rays, and found fluid in his lungs, and his heart was roughly twice the size it sould have been (classic hypertension / congestive heart failure). The vet pretty much said he was a goner. I asked about meds so I could relieve the symptoms somewhat, bring him home and let him die here (not in a panic at the vets office).
He gave me a cardiotonic, an anti-hypertensive, and a diuretic. They were gonna let me go with that, then I asked if it would be a good thing to do LAB, so they could track the effectiveness of the drugs. The vet said, "Yeah, OK, might as well......'
I found some treats to bury the pills in so the cat would take 'em, and after a few days, he was getting perky again, and his lungs were clear / nearly clear.
Then the Vet's office called in a semi-panic: They said it wasn't Congestive Heart Failure, it was HyperThyroid. The hyperthyroid jacked his hormone levels to a high enough level (13.2, normal high is 4.5) that it caused all the other problems. They said to immediately stop the other meds and to instead give him an anti-thyroid. I picked up some anti-thyroid from the vet, put the cat on 'em, and in a day he was his old self.
According to some information on the Internet, hyperthyroid is very common in cats over eight years, is usually treated with pills, though there is a radioactive iodine treatment (~US$ 1000.00, and one to two weeks at the vet), and surgical options.
It's now been about a month on the anti-thyroid, he's playful, lots of energy, and by any visible measure, as healthy as he was ten years ago (he does still have the enlarged heart, and may need to go back on the cardiotonics later).
Anyway ... see if they did Lab, and if not have 'em check for hyperthyroid. The older the cat gets, the more likely the cat is to have it.
Good Luck, sorry to hear about your critter.
Scott