No heartbeat at 10 weeks.?!

I went in for my second appointment yesterday (which I felt was a waste). They took my urine, weight & blood pressure. Said everything was good. My doctor then tries to hear the baby's heartbeat using the Doppler. She tried for what felt like forever but still no heartbeat. It didn't seem to bother her as she was like " we'll try again in 4 weeks it maybe too soon" & that she doesn't usually do ultrasounds until around 18-20 weeks... I'm freaking out bc I just paid for a private ultrasound at 8 weeks which I seen & heard my baby's heartbeat so hearing nothing at all 2 weeks later makes me so nervous 😩. Is  is normal to NOT hear the heartbeat at 10 weeks with a Doppler ? Or should I be really concerned