What Causes Vaginal Bleeding During Pregnancy?

I should be 7 weeks pregnant and have been bleeding for a week. It started off brown spotting, then it turned into dark red blood on Friday. Saturday early morning I passed some tissue about the length of my finger and the bleeding started to go back to brown blood until today it turned bright red, but not that much in quantity. It was never enough to fill a pad and I had minor cramps, nothing too painful. I don t go into the doctor until the 15th and they said if I m not filling a pad in an hour, then not to worry. I just would like to know if this is most likely a miscarriage, or if the baby could possibly be ok even after passing tissue.