What Causes Vaginal Bleeding During Pregnancy?

Hello. I recently found out I was pregnant, and I had some red bleeding less than a tea spoons none came out on a sanitary towel only when I wiped. There was a small stringy clot. I went to the hospital where they did and examination and my cervix was still closed. I then went for a scan and they had to do a transvaginal scan and all they could see was the sac which mesauserd about 5 weeks and 3 days.. no baby. I have to wait two weeks before returning to have another scan to see if there is a baby. Since this I ve had brown bleeding, not I much bleeding but more like very small brown clots. Like coffee granules, no blood. I m not sure what s going on?