I’m scared and confused

Hi guys, so i found out I was pregnant about two weeks ago and I found out from the doctor and the following week I was scheduled for a transvaginal ultrasound. When I received the sonogram screening I was supposed to be about 6 weeks and 3 days according to my last menstrual cycle but I was measuring for 5 weeks and 5 days. This was on September 17th. The doctor told me that this measurement was normal because my average menstrual cycle was 31 days and not 28 days. So now I went to the doctor again and she’s telling me that although she sees a flicker (the heartbeat) she needs to hear one. If I’m only 6 weeks and a couple days is this true ? Should you really hear a heartbeat from a sonogram or is it too early ? This doesn’t make sense to me. Thank you guys in advance.