Besides an incorrectly calculated due date, what does it mean if i am measuring small?

Growth retardation: If you measure small, it could mean your baby is a little small, or it could be growth retarded, transverse lie, or low amnionic fluid. The next step should be an ultrasound and estimation of fetal weight and amnionic fluid volume.

What did your OB say: Either the measurement is incorrect or the baby is small for dates or u do not have enough amniotic fluid, or the baby could be fine, your OB can order an ultrasound that would be the standard of care so u know for certain ! measuring is just a rough estimate.