Spasmodic croup comes on suddenly, often in the middle of the night. Your child might wake up gasping for air. He might also be hoarse, as well as have stridor and a barky cough. Fever isn’t common with spasmodic croup. Doctors believe it may be caused by an allergy or reflux from the stomach. That happens when contents from your baby’s stomach move back up into his esophagus.