It's very possible that you have post nasal drip. Your problem seems to sound a lot like what I have, as well. I've been to my regular doctor, my allergist and my ENT and they have all said that I have to keep using the prescription nose spray (something like Nasonex or Nasalcort or whatever), because that's supposed to help the Post Nasal Drip.

To be honest, it never really helped me. But they all 3 told me that it was supposed to help, so I don't know. I'm not really sure.

Anyway, maybe try to visit an ENT who would be able to check out what's going on. Definitely an ENT would be a good person to see to help you diagnose the problem.