When I said, "that definitely was an early ultrasound, the embryo isn't even visible if you're in your 4th week," I meant that any much earlier than 5 weeks, people sometimes go for an ultrasound and can't even see the baby yet. It was nice that you got a good clear reading at 5 weeks 4 days, because the accuracy that early is usually pinpoint to the day.
Don't worry, sweetheart, you are not pregnant by anyone from the 15th of September. You wouldn't even be pregnant by someone from the 15th of October. Take your boyfriend to the scan, everything is all right.
The pregnancy date count begins on the first day of your last period (or if you don't know what that date was, by an assigned first day of your last period), based on the size of the embryo when seen and measured during the scan. To say that again, medical dating of pregnancy does not start with the presumed date of conception, it begins with the presumed date of your last period. When you had a scan on December 6, the ultrasound saw your baby and said "Rosie's pregnancy is 5 weeks 4 days along." To get to a conception date, subtract two weeks. I.e., a 5 w 4 d pregnancy is 3 days and 4 weeks after conception. (That definitely was an early ultrasound. The embryo isn't even visible if you're in your 4th week.)
Anyway, counting back 3 weeks 4 days from your December 6th ultrasound puts conception at around November 17.
Take the guilty conscience and blow it to the four winds, it is not the September guy's baby.