It is unlikely due to the endoscopy
as the entry for the scope is NOT via the nasal passage, unless you had a nasendoscopy via ENT previously.
Allergic rhinosinusitis is a common cause for runny nose and antihistamines with nasal sprays is the answer. Non-allergic rhinitis
also presents in the same way. One other cause is CSF leak that your ENT specialist
should be able to diagnose for you (need a MRI base of brain).
I would suggest a trial of long-actinh antihistamines with steroid
nasal sprays for 3 months.