Late ovulation

Hello, so I am pregnant with my 5th child. According to my LMP I should be 10weeks which was August 12. I went to the doctors the other day for an ultrasound and to get a due date and they told me I’m only measuring to be 5wks! I’ve never had this happen. So the doctor tells me it’s either that I ovulated late so I didn’t become pregnant until like the end of September which will make the 5wks correct or the baby stop growing. I’m currently waiting for my next appt in 2 weeks to have another ultrasound done but I am an nervous wreck. I haven’t had any signs of a miscarriage so I’m thankful for that. I just wanna know has this ever happened to anyone before