you mentioned that your father in law underwent valve replacement
3 weeks ago,
his symptoms of gastric fullness and sweating after food intake signifies that his general condition is not good.
You have not mentioned about his pumping function, if he had any LV dysfunction pre operatively , which may cause heart failure
So these symptoms may be because of heart failure , so consults a cardiologist immediately so that he may evaluate with ECHO test and advise accordingly.