I always joked that I didn't want to know my due date. Those dates seemed to come and go for my friends and which made them very frustrated! I didn't want to be focused on a date. I am also the odd one that didn't find out what we were having.
But seriously, according to the resources below, a due date is only an estimate of when you will deliver. Also, the fundal height is used to get a rough idea of how far along you are.
Congratulations! I am sorry this has you concerned but it's difficult to guess what they could be calling about. You may want to post on our 1st Trimester Community to connect with other moms-to-be in your same stage.
Here are a couple of resources that may be also be helpful -