Is pitocin (oxytocin) always necessary after section? Doesn't nursing cause uterus to contract?

Usually and yes. Pitocin (oxytocin) is almost always used after a cesarean delivery to insure that the uterus stays contracted and minimize blood loss. Nursing causes the body to produce it's own Pitocin (oxytocin) which will also cause the uterus to contract and decrease bleeding.