I change my ducks' food daily and have impeccable husbandry and still got a respiratory infection.

I'm sure it is very unlikely, I was trying to list all the reasons I have seen my ducks panting. I actually remembered another one - stress. It also seems unlikely that your duck would be stressed for so long.
One of my ducks has very poor heat tolerance and starts panting at about 75 (or lower!) degrees. The others seem fine until 80 ish. They are all wimps about heat. But the panting always stops as soon as I get them in a cool pool in the shade. So if you have her in the a/c I think you can cross that one off, too.
I guess after that it leaves reproductive issues? Is she on layer feed?