What Causes Brown Vaginal Spotting And Abdominal Pain During Pregnancy?

Hi I had a low abdominal pain along with pink to red spotting only when I wiped on 13/12 and 14/12 my hcg level on 13/12 was 540 and on 15/12 it dropped to 511. But since 13th the pain has subsided and is on and off. The bleeding has turned to brown since 13th. My lmp was on 2/12 to 6/12. Ultrasound detected nothing as I m too early in my pregnancy. Is it possible I m not miscarrying looking at the brown spotting and no real bleeding that soaks up the pad.