What Causes Prolonged Periods After A Miscarriage?

My wife had a miscarriage about 3 months ago and had bleeding for 2 months it stopped and started again 1 week ago very very heavy. She has been very tired for the last 2 months barely wanting to get out of bed and she's not that kind of person. She is also just 23 years old. Went to emergency room and they ran blood and said that looked like low potassium caused the bleeding. Does that sound right? What else sounds applicable?