Last period vs measurements

Madison

Hi all! I am pregnant with my second child. My last period started on 09/08/23 and I had my first appointment today. They told me I was about 11 weeks and 6 days. They did the ultrasound and double checked with me on my last period dates and said baby is measuring at 6 weeks and 4 days. Baby has a strong heartbeat but I’m scared because that’s a huge difference in due dates and GA. Do I need to be worried? How could this happen?