Keep getting BFPs after doctors say MC??

Ok so on Thursday my doctor told me my blood didn't have the pregnancy hormone in it so she said I had a miscarriage around 4 weeks because in the ultrasound my baby was only 4 weeks when I was supposed to be 9 weeks. Well I took a pregnancy test today because my sister-in-law said her friends have had doctors tell them they had a MC and they didn't. Well my test was positive. Super faint but the line is there. Idk what's going on? Does anyone have a clue?