Idk what to think 😭

Went to the ER this morning for an unrelated issue, they ended up doing an ultrasound because I haven't had my confirmation one yet (it was scheduled for tommorow) based on my last period I'd be 7 weeks 3 days, but I was on nexplanon for 2 years so idk how regular my periods are after having it out.... Anyway, they have the baby measuring at 6 weeks and the sac measuring at 5 weeks 5 days ( which apparently is what they go based off of).... The doc at the hospital said everything with baby looked good, but was worried because the babys heart rate was only 90bpm they said it was low for how far along I am, they called my obgyn and then told me I need to call and get seen right away..... After leaving I called my obgyn who I trust way more. They don't seem concerned and are pushing my ultrasound to January 31st to give the baby time to grow.... My obgyn thinks it's because I am so early, but I can't help but to worry that something is going to happen. 😭 I just want to know if my little peanut is going to be ok.... Plus I find it amazing that I took several test on Dec 28th that were positive, I would of only been 3 weeks 1 day at the time.