What Causes High Blood Pressure Post Having C Section?

my daughter inlaw had an emergeny C section due to preclampseia. That was Tuesday the 13th and she is home now with blood pressure medicine. She still has some issues with high blood pressure and her feet swelling. Is this normal and will it be a few more days before her blood pressure stays at a normal level. She has never has any blood pressure issues but this is her first baby and she is 36.