Scared and confused

13 days late and took a pregnancy test. It came out positive. I started bleeding the same day. Went to the hospital to make sure everything is ok. They told me that my HCG levels are only 31 and measuring at a week and they wanted me to go to my obgyn in 48 hrs to see if my levels are rising or dropping. I'm scared I am pregnant for my 3rd child and this didn't happen for either of the other 2. Has anyone else ever had this happen to them? If so please help me understand and cope with it