K.M.
My son had a barium swallow at 4 months to confirm whether he had reflux (he did). He drank the liquid and was fine during the procedure. But in the middle of the night, he woke up with a horrifying shriek. He screamed and writhed all the next morning. We took him to the emergency clinic where he continued shrieking. Turned out he was so constipated that he was actually plugged with what looked like cement! Once the doctor pried it out he stopped shrieking, but we had to give him suppositories for several days before his bowels returned to normal. No one warned us about this, so we had no idea what was happening to our poor little guy. I'd recommend giving him a laxative a few hours after the swallow and stocking up on baby suppositories. Good luck!