Shortness of breath after waking

My mother is 63 year old and has suffered a stroke 3 yrs back caused by High BP. She is weak and has spacity on the left side. Now the problem she has been facing for a year now is the shortness of breath after waking up in the morning while lying down and once she sits up the feeling goes away. Visited to two Doctors who have given her Anti-Acid meds which still dont work that well and even her ECG test was fine. What could the problem be? She also has the feeling during her afternoon nap which starts after 1 hour of lying down.