my baby's heart rate 6w2d

Selena
Saw my baby yesterday was supposed to be 7w4d but they said I'm actually 6w2d the heartbeat was 75 bpm and I'm a little worried its baby #1 the tech said sometimes people don't hear a heartbeat that early and it should be higher in a few weeks but that if I was actually 7w4d she would be concerned  last period was sep 8 tech also said my periods are irregular she said she can tell because of my ovaries even though I get them every month