Allergies...why am I not getting better?

I've had post-nasal drip and a sore throat for over a month. I did a week on antibiotics and when that didn't work my doctor thought it might be allergies. For over a week I've been taking Zyrtec-D, Benadryl, Flonase nasal spray and Prednisone. I don't feel any better, in fact today my throat hurts so bad I can barely swallow. I had a throat culture which came out negative. Why am I not feeling any better? What could this be?