Hello, I Have A Pet Related Question. Our Cat (11

Hello, I have a pet related question. Our cat (11 years, 19#, female, indoor healthy cat), has a skin allergy. It has bee determined to be a contact allergy, air born, from deteriarating "stuff" from the Florida everglades. It is atopic flare. She has bumps on tail and lower back that are red scares, folliculitis. She can't reach to scratch.
She got a shot of antibacterial Covenia on 1/22/16. We just received an antihistamine Hydroxyzine to give her twice a day (syrup HCL 40mg).
Question: Do we have to give her this med with side effects and how long is she suppose to take it? Its been one week and she still has the bumps post antibiotic. We wish there was a holistic solution - is this real serious? Thanks, Becky