Home Doppler question

I recently got a sonoline b Doppler to use at home and I have successfully found baby’s heartbeat twice. Both times around 153. My question is both times when I found it, it only picked up for a few seconds before dropping and disappearing. I’m assuming this is because baby is probably moving? I’m only 9 weeks so I also know it’s early to detect heartbeat with a Doppler. Any insight? I’m not worried about it, I’m sure everything is fine, I’m just curious what everyone else’s experiences were.