Krissy, generally implantation occurs 7-10 days after conception. At the time of conception you are already considered to be 2 weeks pregnant. Most of the time your dates will be worked out from the date of your last period, but if they are irregular then it's not an accurate way of doing it.
If you had implantation bleeding and are considered to be around 5 weeks, then this means you would have conceived about 3 weeks ago. The scan is definitely the way to go to work out how far along you are. Your recent scan sounds like mine - just a sac showing and I was 5 weeks.
Congratulations! Good luck with your next scan... there will be a little flickering heart beat there by then which will make it worth the wait!