Postnatal drip is always due to overactive sinus discharge, usually due to an inflammatory response, probably allergic inflammation. The Gastroesophageal Reflux could be an incidental comorbidity.
Next Steps
A plain coronal CT scan of your paranasal sinuses should confirm my diagnosis. Meet a good ENT Surgeon in your neighborhood to help you out. This app will help you find one easily.
Health Tips
Go through the feedback reviews to guide you. For a patient to sit down an type in a proper review, either he/she should be extremely satisfied by the doctor or extremely dissatisfied. Both of which will help you understand the doctor's clinical acumen and manners better.