My 7 Month Old Is Having An Allergy Fit And

My 7 month old is having an allergy fit and has had non stop sneezing all day. Her eyes are puffy And her nose is non stop running. I've done celine, vic's, and humidifier, and it's not helping. Am I able to give her Benadryl? Is so how much, she weighs approx. 15lbs