8 week ultra sound measuring behind - HELP

Gaelyn • Rainbow baby Stevie Grace born Feb ‘18 after 3 losses. TTC number 2. Wife of Chris. Stepmom of Zoe (13) Pitbull mom. Lover of rainbows, sunsets, summer, adventure.
Hi ladies. I'm hoping someone can help me not freak out completely! 
Just finished my 8w3d ultrasound. Saw the heartbeat which was great, but baby is measuring smaller than expected. (The info on the ultrasound said 2weeks behind). 
So we are going back in 2 weeks for another scan. Dr said she's not too worried, it could just be that our conception date is wrong, and she will adjust the due date at the next appt. But I'm obviously terrified as this is often the first sign of an impending miscarriage. (I've had 2 miscarriages before).
 Has this happened to anyone else and the scan measurements been off and baby turned out ok? Or should I prepare for the worst? If anyone can help please let me know cos I'm panicking in the biggest way!!