Slow rising HCG

Started bleeding on Friday the 13th, thought I was 5 - 6 weeks, no cramping, only blood when I wiped went to the hospital as this is my 5th pregnancy and never had anything like this before, was told to go back Monday for a scan. Couldn't see nothing on the scan they thought maybe they could see a 2mm sac but wouldn't confirm it, did my bloods HCG was 505 had to go back 48hr later, HCG were 801 told to come back on the 20th HCG was 1150, they sent me for another scan, what they thought was a sac on the Monday was now 6mm and confirmed it was a sac but nothing else visible. The bleeding stopped on the Thursday (19th) still passed no clots or had no cramps, going back now on the 30th for another scan. Anybody have any idea of what's going on??? All information would be great xxxxx