It could be Cerebral Spinal fluid...but usually it is clear and it has a slightly metallic and salty taste. Do you have headaches that improve when you lie down?
To me, it sounds more like sinus fluid. I would try to see an Ear nose throat doctor. They can put a scope in and take a look. FYI, it does not hurt because they numb it first.
Horselip
Hi,
How are you? Are there other symptoms present? Viral infections such as colds may lead to post nasal drip as well. Drink plenty of fluids and if this persists, you may need to have this checked by your doctor for proper evaluation. Your medical history and direct clinical examination are important. Take care and do keep us posted.