Fetal lung fluid & urine make amniotic fluid, right? So what caused oligohydramnios?

Decreased perfusion. Oligo can be either loss of fluid or decreased production. A stressed infant will shunt oxygenated blood to more vital organs, less renal perfusion, therefore less urine produced. Anything that decreases placental perfusion can cause it (abruption or placental separation, poor implantation, vascular problems, clots, uterine abnls, etc). There are too many causes to list here, ask your ob.