14wks cant hear the HB in doppler help

hi mommies . Last sep 28,2012 i had my Ultrasound and Im 10wks and 4days that time the baby is fine we saw the heart beat and he do a lots of fetal movement , but this oct 17 2o12 i go to my ob because i had this light spotting that goes fast, and my ob said that im 13wks they use doppler to hear my baby but they dont hear it im so worried pls help me to ease this anxiety :)