Ultrasound at 32 weeks. Due date was Sept 12th and changed to Sept 10th. Does this mean I conceived earlier or is it based off babies measurements?

Measurements. Your first due date was likely based on your last period or some very early ultrasound measurements. Both are fairly inaccurate. As you progressed, your due date was refined based on later measurements. Remember, though, doctors (like anyone) are not great at predicting the future. Only about 5% of babies are born on their due date. Hope this helps!