As others have said, diagnosis over the internet isn't always the best way to go, but you sound just like me. I have terrible post nasal drip and terrible seasonal allergies. I also get a bad head cold every 8-10 months. And as one other poster said, it takes me about 1 to 2 weeks of being not sick before diving is comfortable. I've been to a diving specialist ENT and he recommended me up my dosage of fluticasone propionate from 1 spray a day on a regular basis to 2 sprays, twice a day starting 2 weeks out before a bigger dive trip. Then to use a saline spray the morning of the dive as well and afrin 30 - 40 minutes before a dive.
When this first happened to me on my last big dive trip, I had been sick about a week before the trip. My pain was SO blinding I cancelled the last 2 days of diving.
Good luck!