Hey!
Congratulations for your determination and stamina!
I am pretty sure it a post-nasal drip (mucus that your body produces around the adenoid tissue on the back of the nose, to rid of toxins and infections).
If you can spit discretely in a tissue it would be better. It takes some practice to do that. I learnt that many years ago practicing the Ayurvedic Neti cleanse of the nose and sinuses.