Why would I need a follow-up sonogram?

My dr's did a internal sonogram on me last week to determine how far along I was. The dr said she say heart movement and everything looked fine. I just received a call saying they wanted to do another sonogram in two weeks. What would be the reason's behind this?? The dr on the phone did say everything looked good. But once I got off the phone my mind started going crazy and wondering what the reason would be behind needing another ultra sound. Any help would be greatly appreciated. I am between 6 and 7 weeks pregnant. Don't know if this has anything to do with it, but this is my third pregnancy and I had two previous c-sections with a lot of scar tissue.