Please help.

hey everyone. i just went for my frst scan today. by date of last missed period i am 6 weeks 4 days pregnant but scan showed that i am only 5 weeks pregnant. it showed only an empty sac. doctor has advised me to come again for a scan in 10 days so that they can again look for a heartbeat. i am so worried. she said it might be possible that i concieved 2 weeks late. in that case its normal otherwise i might be in problem. please help.is this "two weeks difference" a big problem? is there anyone in the same boat or been through it in the past. share your experiences plz.