Measuring 3 weeks earlier

Hi everyone I’m due March 17 2020 and went for a scan last week after cramping and some spotting to be told there was a heartbeat but it was slow at 108bpm and baby was measuring at only 6w1d when I was 9w1d I’m so worried for a appointment today but my dates definitely weren’t wrong I did a positive pregnancy test the day after my period was due and also had a scan at 5w3d which showed a sac but no feral pole yet and no heartbeat then also they did say my dates might go back a little but nothing to worry about 
I’m so worried has this happened to anyone else and was everything ok