Lost pregnancy symptoms on week 13!

ddh2012

im currently 14 weeks pregnant.  i had an ultrasound on week 6 after having some bleeding and they found the heartbeat.  but now i have lost almost all of my pregnancy symptoms and not scheduled 4 an ultra sound till january 5th .  i had some lower back pain a few day ago and just worried.  the doxctor tried to get a heartbeat on week 13 with the noise thing n couldnt find 1 she said that my heart was so loud and strong that she couldnt hear it .  and again tried yesterday and nothing. should i be worried.  i had a little spotting on week 6 but non since1