Confusing HCG levels - early pregnancy

knottief59b90a31c7d9f61

I am hoping to find someone with a similar story / situation. I had a positive pregnancy test at the end of August. My initial hcg levels were in the 170s. They then dropped to 91 and then 32 in which I began to have bleeding (nothing more intense or heavy than normal cycle bleeding). The bleeding / spotting stopped after a week and when I went back to recheck my levels and make sure I was back to normal range, they went up to 47. From there they continued to rise to 134, 303, 434, and today 732. They are not rising as quickly as my doctor would like, but my ultrasound last week did not show any signs of ectopic neither signs of a healthy baby as my numbers are still much to low. I was originally told I miscarried, then the possibility of ectopic, now they are saying this is a pregnancy of unknown location and abnormal. I was on letrozole before getting pregnant to help with our recent infertility. I am now wondering could this be a vanishing twin that I experienced? I lost / miscarried one but a second is still viable with my decrease in numbers and bleeding but now an increase again. Has anyone else experienced anything like this? Any input is appreciated! I am just still so confused and baffled