Could Vaginal Bleeding During Pregnancy Cause Miscarriage?

i went to the er the other night due to vaginal bleeding and the did a ultrasound and said that the baby was good and that my levels where great i heard the heartbeat i was eight weeks one day at that time the bleeding only lasted a day i think that it was the time that i would have missed my second period i dont know it did it last month on the second brown spotting and then the 5 bright red spotting but then it stopped the doc in the er told me that i have low potassium and sodium for which been drinking ensure and v 8 i dont have much of an appetite am i going to miscarry the heart beat was 167