I had my first ultrasound at 8weeks 4days. I am 9weeks and 1day today.

When the doctor said the heart rate she said it softly and quick. I heard 75 I think, But that seems way too low. I'm only 17 and this is my first pregnancy, no history of miscarriage. I'm so nervous that my baby won't be heathly or that I'll lose the baby. The doctor did say that everything was good and right on track, but I just don't understand the heart rate. Could I have just missed heard?