Ugh due date not matching ultrasound

So my last period was sep 11-16 and I had sex on the 16 and took a morning after pill and turned around and had sex on the 24. My doctor assured me that I didn't get pregnant from having sex on the 16 but I had a ultrasound last night and the are saying I'm 6w 6d which would add up to the 16. Has anyone went threw this it's driving me crazy