S.Y.
My sister went through the same situation. She went in with the doctor thinking she was about 7 weeks along, but really was only 6 and could not yet see the heartbeat. She went in again about a week later, and they were able to see it. She had previously had a molar pregnancy, and did not find out until she should have been 11 weeks that there was no heartbeat. Needless to say, she is EXTREMELY excited and relieved that there is a heartbeat and she just entered the 2nd trimester and things still look good. Try not to stress too much, it was just a little too early and they should see it at the next ultrasound. Best wishes! and keep smiling :)