We went to er for severe allergic reaction. thought it was from antibiotics, but now wondering if it might have been from a single dose of flonase?

Prob'ly not Flonase: Allergic reactions are rare to corticosteroids like Flonase; indeed, corticosteroids are among the main drugs to treat allergic reactions. One of your antibiotics is a far more likely explanation than Flonase. However, if your allergic reaction was primarily a breathing problem or upper respiratory blockage, check back with the ER and make sure they took the Flonase into account.

Highly unlikely: to be from Flonase which , to my best knowledge, has never been reported to have caused severe allergic reactions. Irritation perhaps but allergy unlikely.

Unlikely: Flonase allergy causing severe allergic reaction would be very rare. Make an appointment with a board certified Allergist so that he/she can take a detailed past medical history and go over the events of the day of the reaction so that they can get a better sense of what you might have reacted to.