Fast fetal heart beat?

i had a doctor's appointment yesterday, and she checked the baby's heart beat. to me, it sounded very very fast and all she said was "he is active". however, after that, she scheduled me for a sonogram appointment for 5 days later. i am 33 weeks along, and have no need for another sonogram. my doctor tries not to worry me unless there is a real reason, so she didn't really say much about it, but i am still worried. why would she schedule me for a sonogram...and so soon? has anyone dealt with anything like this before? thanks in advance! :)