Hello and welcome to the community.
I agree, you should see an allergist. There are various types of nasal sprays that they can prescribe which you can't get over the counter. Also, they can test you for allergies and start you on immunotherapy if needed.
You may find that a sinus rinse helps your symptoms twice a day (up to five if needed). Also, showering before bed or after being outside for a long period of time may help as well. If you suspect that you may be allergic to dust, try encasing your matress, boxsprings and pillows in allergy proof cases. Washing your bedding in hot water and keeping pets out of your bedroom will reduce your allergen exposure as well.
The best way to control allergies is to limit exposure. Hopefully the above measures will help you do that.
Another thought is GERD. GERD (reflux) can cause inflamation of the sinuses causing PND and sinus infections. A person can have GERD and not experience the typical heart burn they would expect. You should talk to your doctor about that posibility.
God bless.
You may want to see an allergist and have some tests done to see if you are allergic to something that is causing this post nasal drip (PND) and then the cough ... so sorry to hear you are enduring this. C~