When is oxygen needed? My mom is 78 years old; trouble breathing when walking, but fine when sittin

Recently diagnosed with Lady Windermere syndrome and recovering from pneumonia. Doctor says she doesn't need oxygen but she seems to be having great difficulty breathing when trying to walk around. As long as she is sitting or laying down, she can breathe. It seems to have progressively gotten more difficult these past 2 days. Should we consult another doctor? I told her to call 911 if her difficulty breathing becomes worse.