Length of time for hCG level to return to zero

Hi,
I found out on 8/25/16 at 19 wks 4 days pregnant that my baby girl had passed. I was admitted to hospital on 8/26/16 and gave birth on 8/27/16. On 9/6/16 I had sex which was 11 days after giving birth and again on 9/12/16 which was 17 days after giving birth. I took a pregnancy test today 9/17/16 which is exactly 3 wks after giving birth and got a very visible positive. My question is, could this be old hCG from the miscarriage or a new pregnancy? My breasts seem to be getting more tender as it had almost subsided.

I would like to also add that a few days following the 1st day that I had sex which was on 9/6/16 I started having sharp pains in my right ovary which only lasted a few hours and then started discharging EWCM (egg white cervical mucus) for about 2 days. I’m not sure if I was ovulating. I am aware that I would not be getting a positive reading from when I had sex on 9/12/16 as that would be to early. The date more so in question is 9/6/16.