Lush - dont frighten yourself. Dating based on LMP is not accurate especially for those with irregular periods. When i went for first consultation, my gynae thought I should be 15 weeks based on LMP but actual scan revealed 10 weeks. So does this mean my baby was lagging by 5 weeks?? Of course not.
Dating by you/s although not 100% cos' sometimes it can be +/- 1-2 weeks. That explains why sometimes the EDD keep changing by a few days when you go for scan during diff appointments.
From your post, it seems that baby has healthy heart rate. What does your gynae say? If he thinks baby is growing well , then dont worry too much ok.
I know you feel worried due to past experience but the last thing you need is negative thoughts.
Think positive and keep your mind calm. You're almost end of 1st tri and Im sure preg wil be stable. Relax..