What Causes Lower Back Pain During Pregnancy?

I went to the doc tue. For lower back pains I just found out I was pregnant about two weeks ago they tested my urine then done a ultra sound they said I was measuring 6 1/2 weeks and their was a pole but no heartbeat also said my sac was not shaped normally, they doc was not very sympathetic and said I may start bleeding.i go back next thur. For another ultra sound can you give me and helpful advice I have been researching and im very nervous im still getting pregnancy symptoms and feel fine I have heard lower back pain can be considered normal.