Help!! Spotting early.

Ok so here is a little bit of history. I have been having dark but light bleeding since a few days before my expected period. It’s kept on daily since then. My expected period was Sept 1st I took the pregnancy test the day after and got two positive results and earlier this past Monday I had it confirmed at the health dept. My last period was August 4th. So I guess I am around almost 5 weeks. I called my OB yesterday to tell them about the bleeding as I registered as a new patient and his nurse told me I needed to go to the ER when I mentioned I thought I may be - blood type. So I went to the ER and they did a vaginal ultrasound, checked my blood type and HCG levels. After my results came back they assured me I was indeed RH- and administered the Rhogam shot and then told me they didn’t see anything on the ultrasound other than my ovarian cyst. They said my HCG level was 236.2. He says that number was low but could be because I’m still early in pregnancy or worst case scenario an impending miscarriage. He said going from what should have been my ovulation date I’m technically only 3 weeks. Which makes more sense to me. Why would you count from the first day of your last period when you weren’t technically pregnant at that time. Has this happened to anyone else? I have to go back to my OB Monday to get another HCG check to see if my levels have doubled or not before they know anything.. any help is appreciated.