A lot of women measure "small" but that doesn't always mean somethings wrong hun. Baby could be perfect size or even just a petite size. I bet everything is perfect and your little one is growing healthy.
Try not to stress. I measured small my entire pregnancy. Ended up the placenta had died. Had my little boy at 36weeks 100% healthy! He was just really tiny! Praying for you! Just remember whatever the issue your doctors are there to help!