Placenta bleeding??

Leslie
I am very early in first pregnancy, 6 weeks, and I had a sonogram yesterday. My doctor told me today that I have bleeding behind my placenta. She's not worried but then she said it could lead to a miscarriage! Has anyone else ever gotten this news?? She also said the baby's heartbeat is between 118-120 and that is low. But when I looked it up it was normal. Can anyone reassure me please!!!