Congestive heart failure. Cough under stress - heavy cardiac load.
My grandmother died at 83 years old from congestive heart failure, she never coughed, not once..but she ballooned because of all the fluid retention(Coumadin after a long period of time no longer worked) my guess is this coughing is coming from her Esophagus, she might have Barrett’s Esophagus which can become Esophagus cancer after a while. I doubt she has Esophagus cancer because if she did her death would come pretty quick you dont survive long with that