35 weeks measuring small

I went to my doctor appointment today. They've been telling me for a while that hes measuring smaller. But today the doctor did my belly measurements and said hes measuring small. I asked about how much do you think he weighs and she said probably around 4 pounds. She didn't seem worried about it but i am. Also she tried feeling for his head on my abdomen and said she couldn't feel it. But I've been told at all of My other doctor appointments they think hes head down. Ill be 36 weeks Thursday and im getting worried.