Persistant dry cough. Chest X ray results....?

Had a pesistant dry cough for 3 moths, got chest x ray results back and all clear. Was working on a farm in VERY dusty conditions prior to bad chest. Doctor said chest is ok. Told me to buy antihisimines and that would stop the cough, I told her that im no longer exposed to dusty conditions and she still told me to get antihisimines. I also have to keep clearing my throat as if something is stuck there. Do you think its post nasel drip? What medication should i get?