My Last Period Was Sept 26-30 I Found Out I

My last period was sept 26-30 I found out i was pregnant on oct 28 but my due date is june 25th instead of july 2nd is it possible that I concieved 3 days after period ended ?Had an ultrasound at 14 weeks 2 days but baby measured 15weeks 2 day thats how my due date got moved up a week ahead should i go by lmp or dated scan how accurate is the date scan ?