Why did they stick a tube down the throat of my 31 week baby then take it out shortly after? that seems cruel.

Several reasons: You can ask the doctor what they were doing, but here are some common reasons. Following birth, the baby's airway if full of amniotic fluid. If that fluid is still present when the baby takes a breath, they can suck the fluid into the lungs. A suction catheter is used to removed the fluid. At 31 weeks, the lungs are not mature; they could have been giving surfactant to help the baby breathe.