What Causes Brown Vaginal Bleeding During Pregnancy?

Hi doctor, I am sutha.now I am in 8 week pregnant.last 4 days to till now I am having light bleeding with some light brown color.but I dont have any stomach pain and any period pain also.I asked this to my doctor via phone she said to take susten 100 mg tablet for ten days so please advice me doctor.this condition is serious or not.i am waiting for your answer