Trying to guess my due date

My last & only period since giving birth in November 2017 was December 2nd. I was exactly a week late when I tested positive on January 6th. It was faint & on a FRER. I tested negative multiple times before then. Anyways, I tested positive with my son at 3w2d. Based on my period, my due date is September 8th. I’m going to assume more like September 20th, but I have no idea.... I’m supposedly 5 weeks 3 days, but I highly doubt it since I just got my positive. When is your due date if your period was around mine and/or if you tested positive on the day I did. I will add, I started having symptoms right when my period was late, but kept testing negative. So maybe my positive took longer? Maybe I ovulated late?