6 months pregnant and measuring small?

This is my second baby, and it has been completely different than the first. I had a lot of nausea, vomiting, headaches, muscle pains, etc. Now most of the sickness is gone, just pain in arms, right leg goes numb (when standing), headaches, but the part that scares me the most is I am measuring small. Initially I was measuring large for my due date - now it has changed to smaller. I know she is laying horizontally instead of vertical like my first, and the ultrasound and AFP tests came back good. But, for some reason I am more frightened about this pregnancy than I was with my first.