2nd ultrasound, had my dates pushed back to 6w 1d as supposed to 8w 6d by LMP

I got my 2nd ultrasound, baby measured at 5mm, no heartbeat. Doctor said I was only pregnant for 6w 1d instead of 8w 6d according to my LMP. I had my first ultrasound 2 weeks ago at supposedly my 7w but we were only able to see an empty sac so doctor said I was only 6w. We went back 2 weeks later and after the measurement they said I was 6 weeks again. Does anyone have similar experience? How can my weeks be so off? I haven't felt any morning sickness. The only noticeable symptom is tender breast and that even seem to go away sometimes. I have an increase in appetite and I feel my belly is bigger but I think it might just be from constant eating. I will go back to doctor next week to follow up for a heartbeat. I'm really worried though. I fear the baby could be stop growing at some point?? I haven't had any bleeding or sharp cramping. If that's the case when will bleeding occur?