Low HCG levels

Hello. I’m going through a confusing and hard time right now. I took a home pregnancy test on 4/22 and it was a faint positive. Took a blood test the same day and the HCG level was a 6. I then took another blood test on 4/26 and it only went up to 16. I started bleeding heavily on 4/27 and my doctor confirmed over the phone that I was having a miscarriage especially based off of my levels since they weren’t rising like they should. I went in again 5/1 to check my blood and it jumped to a 51. That was super confusing because I was sure that I lost the baby with all the bleeding I did. (Bleeding lasted about three or four days) Doctor did an ultrasound today 5/2 and so far there is no signs of it being ectopic but he is not ruling it out yet but so far I have had no symptoms of an ectopic pregnancy. Took another blood test today as well and it went up to 57. Doctor gave me the option to wait it out until I miscarry, he says this is a non viable pregnancy. The doctor seems confused himself though and I don’t understand what is happening. Has anyone ever experienced this? What was your outcome?