Had Pneumonia, Restlessness, Breathing Difficulty, Swollen Face, Given Potassium, Heart Issues. What Can Help?

My 85 year old mother in law was admitted to the hospital due to pneumonia . On the fourth night of her hospitalization, she became restless, had difficulty breathing , and her face suddenly became swollen. I remember that earlier during that day, she was given potassium thru IV and she was complaining that the medicine injected hurts. We were told by the doctor that she had a heart attack and the reason given to us is because she has a history of heart issues. What could be the reason why she had a heart attack and swollen face at the sme time?