What Causes Vaginal Bleeding During 6th Week Of Pregnancy?

I m currently 6 weeks pregnant went to hospital yesterday morning because I started bleeding like a normal period only cramped after bleeding a little scale of 2 maybe if even that they referred me to come back in 2 days because they couldnt see anything in ultrasound but did see a little blood in ultrasound and couldn t explain why I was bleeding so they said threatened abortion return in 2 days to check if my blood work for HCG levels double I have to go back tomorrow well today about an hour ago I bleed a small blood clot out no pain still but still bleeding feeling like a period I earlier this year did have a miscarriage and I m really worried if that s what s happening now