Irregular period, now pregnant

Hey ladies! So I've had irregular periods since I got off the pill 3 years ago and they range from 30-45 days. The first day of my last period was September 2nd. The last time I had sex was on October 2nd, my fiance works out of town and that's the last time he was home. I went to the doctor and had a urine hcg done on October 16th and it came back negative. I had some spotting from the 18-23 of october with no cramping or symptoms. It was very different from my periods which are usually heavy with bad cramping. I've got a doctor apt in 3 days but I wanted some opinions. If I work it by the last time I had sex I'd be roughly around the 8th week of pregnancy. But I know it's probably wrong.