Help dates

So it's 7 weeks since my last period but only 4 weeks since we BD so obviously I got caught pregnant after that as we only did the deed twice in the same week that month - so surely I'm only 4 weeks pregnant as iv been for a scan today expecting to see baby as its 7 weeks since LMP but only sac & yolk was present but sac has doubled in size since last weeks scan !!!help did I ovulate late!!! Totally confused