The post-nasal drip and the recently diagnosed thyroid cyst are unrelated.
Your age is 64 years. You started using the medicines for post-nasal drip 20 years ago, i.e. after the age of 40 years. Your mother also has some allergic disposition. Nasal allergy is well known to run in families. Sometimes, some of the medicines taken for blood pressure also may cause a nasal drip. Kindly seek the help of your family doctor. He or she can give you immediate relief, by prescribing some medicines.
Afterwards, under his / her supervision, do the following:
Exclude diabetes by doing a fasting blood sugar; repeat the blood sugar two hours after food. Or, better still, do a glucose tolerance test. This is a simple test; however, it requires you to spend about two and half hours in the laboratory.
Take a sinus x-ray to rule out any sinusitis, as a cause or as a complication of the nasal allergy. Occasionally, a fungal sinusitis may manifest as persistent nasal drip and post nasal discharge. Your doctor may even advise you to have a CT sinuses and / or a diagnostic nasal endoscopy. Both give very good information about the sinuses.
Some people have a weak valve between the food pipe and the stomach. Such a condition allows the contents of the stomach to regurgitate back into the food pipe. This is called Gastro-oesophageal reflux disease (GERD). Such reflux may lead to a persistent inflammatory state of the nose, throat and the sinuses. When the above are convincingly excluded, we may have to think of an age related nasal drip. In this condition, sneezing and clear watery nasal discharge occurs. This condition is quite rare and is believed to be hormone related. Treatment with medicines gives good results.