pregnancy problems? whats wrong?

Ok i dont understand whats going with my pregnancy. my last period was nov 5th and i know i write it all down which should make me 10 weeks but yeaterday i went to the doctor for the first time and my doctor said after she did exams down there i feel more like 8 weeks.... so what could be going on? could my baby not be healthy? im scared its not growing right? could it really be something serious? all my lab work came back good she said im just nervous i will have an ultrasound to measure hopefully everthing is ok! will my babyjust be small or is there something wrong? any one had this happen to them? :/