What Causes Fever In A Child Post Tonsils And Adenoid Removal Surgery?

My child had her tonsils and adenoids surgery yesterday morning. We stayed overnight 23 hours before we were suppose to be discharged she had a fever of 101.3 at 12:30 PM I questioned the Dr asking shouldn t we wait before being discharged to identify why fever had spiked. He said no it was normal . We left the hospital she vomitted in the car and and at 4 pm she spiked with 103.6 fever. I threw her in the bath got her down to 100 he said to add 1 teaspoon of tylenol with the already prescribed hydrocodone 7.5 ml every 4 hours and 2 teaspoons of motrin and if continues to have fever take her to the ER for IV fluids. ? Why would he not just keep her knowing she had a fever?