Feeling a little worried

So I had an apt last Thursday for an early scan and apparently I was 7 weeks at the time, the baby measured as 3.3mm when at 7 weeks it should be around 10mm, doctor is putting be backs week to 6 weeks I think thinking I could have conceived later than we thought, but he has me coming back this Thursday coming to check the progress of the baby I guess to see the size for a more precise due date?? , as anyone else been in a simular situation and everything was fine ?? I can't help but worry :(