Is this a Miscarriage

[mumof2Jodi]

I have just found out I'm pregnant again with my third which was planned so we are delighted 😁previously I have had 4 early miscarriages all of them not passing 6 weeks. Because of this when I was pregnant with my second child they sent me for an early scan. This was an external scan and at 6+4 weeks there was a heartbeat visible and I went on to have a healthy baby boy. Yesterday I went for an early scan at 6+2 which they done internally. This time she told me there was only a pregnancy sac but she couldn't see the yolk sac or a fetus  she said either the pregnancy is not viable or that I am earlier than what I think. They have booked me in for another scan in 2 weeks. Im confused because I am very certain of my dates as I have a 26 day cycle bang on and my LMP was 18th June. So my dates are 6+2? Has this happened to anyone else should I be worried? My symptoms so far are dull back ache and some mild cramping. Usually I'm being really sick by now but it's hasn't kicked in yet? With my first 2 it was really bad all day every day till end of my pregnancy and started about 6/7 weeks. I have had waves of it but not as bad as previous. Anybody had empty pregnancy sac at this stage and gone on to have healthy pregnancies?