Most women ovulate one to two weeks after their menstural starts. Thats why docs give a due date and say "two weeks before or two weeks after". Id say just wait until your next sono. When I went for my sono with this pregnancy my LMp was Sept. 28th. I went on November 2nd and was only 3 weeks pregnant. So its normal. Just be patient and wait till the next sono. Congrats on your pregnancy!
All babies grow at their own rates. At the beginning my baby was about 3 or 4 days "behind" where I was at in my pregnancy and now he is 3 weeks ahead (Big Boy!) so I wouldn't worry about it. Normally ultrasounds don't pick up the pregnancy until you are 6 weeks along so if it showed up on the ultrasound then I wouldn't be concerned. Did they check to see if baby had a heartbeat? They usually do and if your doctor was concerned they would have said something about it. Typically your next ultrasound will be around 12 weeks which is the first time baby will look like a baby instead of a grain of rice and they will do measurements again. My baby didn't start catching up or measuring bigger until I was around 26 weeks.
Congrats on the pregnancy!
AT this point, wait and see. It's highly possible that you ovulated later, making the dates by your lmp off. Especially if your cycle tends to run longer than the typical 28 days. Have they scheduled you for another ultrasound? You should wait about 2 weeks fromt he first ultrasound and have another one done. There's still a good chance that the baby is fine, just not as far along as suspected based on your period dates.