That happened to me about 7-8 years ago with that h1n1 flu. By day 3 both lungs were barely working, pneumonia. Only antibiotics saved me. I have never been deficient in vitamin d since that time. I never want to be that sick again.
Antibiotics don’t touch flu which is a virus so you must have had a bacterial component that can occur after the onset of the flu. Remember, antibiotics are only effective against bacterial infections.