I am sorry to hear about the situation. Pus filled lungs, a condition called empyema, is caused due to infection with certain bacteria. It is probably unrelated to the child birth. It is also possible that her immunity dropped with the strain of delivery and she got infected with the pathogen which was otherwise around. She has to be treated with proper antibiotics, the sensitivity to which is to be found out by blood culture. Your English is fine and you need not worry about it so long as you can communicate what you want to say.