Fetal Heart Rate

I’m currently 36 weeks with baby boy #2. My baby’s heart rate has slowly declined at every doctor visit. Last week, it was 115-117. The nurse said she could see baby moving as she was using the Doppler. To be safe, my doctor sent me to L&D to be monitored for 30 minutes. Baby’s heart rate went up to 124-138 and he was moving around more when I was lying flat. I was able to leave after 30 minutes and told I was dehydrated and my amniotic fluid was a bit low. I’m just worried about the heart rate. My first born was never this low. Baby moves a lot throughout the day and I count kicks. Anyone had this?