Question about heartbeat

I had my first physical today at my OB. My doctor looked for babies heartbeat with the fetal Doppler and only got to here it for 5 seconds and told me that's probably all were going to here because the baby was moving. She didn't really stay in place in one spot and keep constantly moving the Doppler fast. I don't know if she was really trying. Anyways, is this normal? I'm 13 weeks and she didn't seem concerned at all and said that the baby is small and moving too much. Has this happened to anyone else?