Small Baby at Scan?

I went for my 20 week scan and they did the measurements of everything and said that it was a little on the small side for every category...

They changed our original due date from 28/12 to 24/12 at our dating scan, but now it looks small for 20 weeks. They want to do another scan in 3 weeks to see how much the baby has grown.

Has anyone else experienced this? I'm worried it could have been my caffeine intake of 1/2 cup of coffee on workdays. I'm also not showing that much either.