It could be sinus drainage possibly from a sinus infection or from allergy.
Have you tried irrigating sinuses using a saline solution? If not, give that a try. I use a mixture of 1 cup boiled water, 1/2 teaspoon plain salt, 1/2 teaspoon baking soda. Stir well to dissolve, strain thru clean paper coffee filter, put into nasal spray bottle. COOL saline solution before using. Lie on back with head hanging off bed and squirt a few squirts in each nostril. Turn head slowly side to side. When getting up say kkkkkkk to keep solution from running down throat. Blow nose gently as needed. My physician approved this for me several years ago.
A few weeks ago a pharmacists wife asked me for this saline solution recipe.