Oh my, I need to put my thinking cap on for this. The issue of knowing exactly when you conceived is that conceiving is a best guess. Rarely can you know exactly when. Sperm lives for up to a week further complicating matters. Did you have sex after September 15? Or are the four times you mention the only times you have had intercourse?
Hi, when you had your ultrasound on November 5, what estimated due date did they give you? It can be less confusing to count back from that than to try to work from a GA ("weeks pregnant" count).