Measuring too "low and small"?

I went to my biweekly appointment yesterday, the doctor checked the baby's growth but had a conceded looked on his face.. he went to check the heart beat but didn't even do that for more than 3 seconds you couldn't even hear the heart beat he did it so quick. He left the room and soon came back saying he was worried because I am measuring too small and low for 33 weeks. We asked a hand full of questions but got no answers should I be concerned??