6weeks YS no heartbeat yet !? Should I be worried !????

Hi guys so I recently just go pregnant again after having a miscarriage last April and trying since then. I went to my first ob appt for my 6week ultra sound on Tuesday where you’re able to see the heart beat and I was told the YS was there but no heartbeat yet. I was asked if my period is irregular which it is I’ve been nauseous so my doctor states that the time of conception could be off and she rescheduled me for an appt next Wednesday no do another sonogram. I am extremely anxious I burst into tears when she said she saw no heart beat off rip. I just don’t think I can handle another loss! But I’ve done research and it states that it’s at the 6.5 - 7 week mark I should definitely see it. I was 6weeks and 3days when I went to my appointment I just wanna know if this has happened to anyone else and what happened ? I’m trying not to stress but that’s hard for me I think to much! If there are any doctors here willing and able to give me advice or feedback that would be great also . Thank you guys!